Wait, I have clarifying questions.

Were you also in the rain?

Were you offering the umbrella to take with me?

Were you offering to share the umbrella?

I don't mind a little rain, especially when I have my rain jacket on. Sometimes I find carrying an umbrella around cumbersome. If it's really pouring, I have several umbrellas of various sizes, but more often than not I'll just go without.

If you are also in the rain, I don't want to deprive you of the umbrella you cared to bring.

If I'm taking the umbrella with me, that's an implied obligation to return the umbrella in good condition, something I don't trust myself to remember to do. I'd rather be a little wet than create that social contract I'm probably going to fuck up somehow and foster animosity.

If you're offering to share the umbrella, there's an implied intimacy there. I'm happily married, and we are secure in our relationship that neither feels threatened that the other would cheat, but I still don't want to give someone else the wrong impression. I have friends I'll happily share an umbrella with, but there's a much larger number of people who might offer and I'll decline. Like another parent at a kid's sporting event. Someone who's name I ought to know, but can't remember, and I've met the other parent, but I don't know what their deal is. Someone who goes in for the hug, but has never seen the inside of my car. Are they just super friendly, or are they angling for more? I'm fairly oblivious to the "signals" as my wife will attest, and I've been accused in the past of leading people on, so I'm a little gunshy.

So if that's being interpreted as toxic masculinity, I think I'd rather people believe that about me than have to politely decline another indecent proposal from someone I'm going to have to see at every PTO meeting.