I'm usually on the "no, that girl who smiled at you because she's a friendly person didn't lead you on" side, but you bought a bed together and do sex stuff, even if you haven't had actual sex...

That doesn't mean you owe them forever, or even now, but you're a big part of the reason for their emotions here.

Start by broaching the separate beds, IMO. Then give a little time before dating, if you want to minimize pain.

You don’t

You have lived with diffuse boundaries for some time and are now reaping the penalties. You can and should proceed with care and grace as you implement boundaries and define roles to move to where you want to be but it is absolutely foolish to think that it will not be at least a little hurtful to your pseudo partner.

they will implement their boundaries in response to changes you are proposing; you have to respect these. If they chose to leave and tell you to fuck off then you have learned a valuable lesson in why you shouldn’t let boundaries be so diffuse for so long through so many changing contexts.

It’s not realistic (usually) to expect you to know everything you need and want from a relationship up front but when contexts change you need to clarify what is and is not okay. If you’re okay with keeping it casual after things don’t work out that’s fine but make sure they’re aware. If they suddenly have to move in consider the boundaries of the situation again: are they still cool with keeping it casual? Are they now that you share a bed? Are they now that you’ve purchased a bed together?

If you’re the one that wants it casual and wants the door open for new relationships it’s your responsibility to make sure your partner is aware of where you stand. One could say your friend/partner is foolish for assuming you’ve changed where you stand, and they’d have a valid point, but one could also say that you’ve been very misleading here. Boundaries need to be enforced and they need to be occasionally reviewed as contexts change, otherwise they fade away

You don't, I'm sorry. It sounds like the only scenario you'd be happy in is if your friend is happy to share you, although from the sounds of your story your friend wouldn't be happy to share you. Which means one of you wants more in the relationship and haven't cleared business about it. I would recommend you talk this with them. Maybe it can work between you two or maybe you realise your friend isn't as dependent on you as you think. If they're clinging to you but you want to move on please cut those strings, they only become more and more painful with time.

I can't help but notice you didn't say anything about how your potential new partner might feel about this. Perhaps you didn't think it was relevant, but that's a huge blind spot if you haven't considered it.

Or try having a relationship with your friend? A relationship is basically a good friendship with shared responsibilities (and sex, if you've got the time). You're almost there already.

You could contextualize the conversation by bringing up how you met and having a retrospective discussion about how and why it didn't work out romantically.

Maybe ask them if they've considered or are interested in seeing other people. This will let you gauge how they feel about it and the current state of your relationship, and also give you an opportunity to bring up how you feel.

Then, the conversation isn't just about you seeing other people but about what's best for the both of you.

So, when's the wedding?

Someone will get hurt, there's no avoiding that. But you should not sacrifice your own happiness for the sake of not hurting your friend. Sometimes we need to be a bit selfish in life.

Bit of a different take from many of the other comments.

Relationships don't have to be the way they are traditionally. You don't have to be monogamous. You can be in two relationships, one of which is romantic, one of which is a strong friendship with sexual aspects. It'd be under the polyamory umbrella. There's plenty of potential partners out there who are ok with or would even want this kind of a constellation.

The very important caveat though: even more so than in any other kind of close relationship, this requires a LOT of communication. You need to clarify with your best friend what kind of a relationship it is that you have. You don't have to label it, but you have to figure out together what you want from each other and what your boundaries are. The latter includes what kind of relationships you're ok with the other one having with someone else. Then, when you date someone else, you have to have the same conversations and be open about your other relationship(s). I'd be upfront about the latter, the former can happen over time.

If none of this sounds like something you'd want, that's of course perfectly valid. The point stands though that you need to clarify with each other what you want from your relationship and what your boundaries and needs are. This might mean having to change your relationship dynamic.

Some context: my best friend and I are super close and find each other sexually attractive, but aren't romantically interested in each other. We've talked about that and keep checking in. We've done some second base things and cuddled. I now have a partner that's decidedly monogamous. Now, I might still lightly cuddle with my best friend, but we wouldn't kiss anymore. Those are all boundaries that had to be talked through.

I considered making an alt account for this but couldn't find a server that let me do so without waiting for approval so whatever.

I've been in a very similar situation. Good friend hit a rough patch, moved in with me, a relationship developed but at the same time I knew I'd want to see other people.

The most important thing here is definitely communication. You say you don't know how they would take it so I assume you've never talked about this. So I urge you to talk to them as soon as possible. If you can, do it today. Don't fall into the trap of waiting too long. I've learned that the hard way in a previous relationship where my partner and I had very different plans for the future and, not wanting to upset them, I waited for a good time to bring it up. Guess what, having something so important in the back of your head will strain your relationship to a point where there is no good time to bring it up.

If you don't want to scare them, start with hypotheticals like "how would you feel if I decided to see other people?" then work your way up. Be gentle but be honest. Tell them that wanting to see other people doesn't mean you love them less or that they are not enough. For me, comparing it to other things I love like a favorite food or hobby worked well. Let's say I love cheeseburgers to the point that my friends call me the cheeseburger guy. No matter how much I love cheeseburgers, sometimes I need something else. Eating a pizza doesn't mean I don't love cheeseburgers anymore and I would never want to stop eating cheeseburgers. Just make sure they know you're not comparing them to a cheeseburger unless they're into that.

Find out what they want. Best case, they are in the exact same situation as you and are just as afraid to tell you they want to see someone. Not that likely but possible. Maybe they are okay with you seeing someone but don't want anyone else for themselves. Maybe they want to change your relationship in a way where both of you are happy being the only ones for each other. And maybe it turns out that your plans just aren't compatible. Be prepared for that. Think about if you'd rather give up on your plans or on your friend. Both is fine, it's your life. Not talking about your wishes doesn't make you more compatible, it just builds tension until things go spectacularly wrong.

And most importantly, respect their wishes. If they tell you that they don't want you to date other people, that's absolutely valid. Don't try to persuade them and for the love of everything, don't do it behind their back. Either don't date other people or make sure to end the kind of relationship you have at the moment. Hopefully you can stay friends.

Finally, about that other thing you mention, other potential partners not accepting that you have someone you share your home and even bed with. Don't worry too much about it. When I started dating again, I noticed that there are surprisingly many people out there who are totally fine with being open, poly or whatever you want to call it. Especially on dating apps. I don't know about Tinder but OkCupid even lets you filter for that. Just be honest about it.

Edit: slightly updated some wording after I read some of your replies which made the kind of your current relationship a bit clearer. The original message still stands.

I have so many questions, but some of the more important ones:

• Is this person of the opposite gender (of whatever gender you're supposed to be attracted to)? I assume so, but do correct me otherwise.
• What does this person think of the nature of your relationship with them? Are you two clear that it's plutonic?
• How could you share a bed and not develop feelings?
• You care so much about this person, how much does this person care about you?

Eat your best friend op.

/s