How many comrades of the past met each other through the Soviet Party or the Communist Party of China or in Cuba or Vietnam? Probably millions.

I would not be so sure. From what I have read party members in the Indochinese/Vietnamese Communist Party explicitly described themselves as having a sibling-like relationship with each other and sleeping around in the party would cause scandal. At least one man was executed by other party members when a woman comrade reported him, though there might have been some SA behavior going on in that case. I'm pretty sure that the CPC was similar in that regard too. I also believe Trotsky had somewhat of a "reputation" within the Bolsheviks for that sort of thing, which seems to indicate it wasn't that common there either (or that could have just been made up one of his many haters).

Not saying it never happened in history but ultimately people should be there to work and it's the same problems with dating at work, and those problems multiply when you're trying to run the party as an honest to marx people's liberation army. It did seem more acceptable later when those parties transformed into government bureaucracy though.