It's pretty similar to most other roleplay in an RPG in my experience: you remember that you're all playing characters and you respect out-of-character boundaries (including fade to black for anything too explicit or indulgent). I've been worried about crossing boundaries before but I don't think I've ever actually crossed a line over the course of quite a few campaigns, even with the couple of times I've had to fade to black. You just have to know your players and be explicit about boundaries; I've had players request that no romance be present in a game before and I've accommodated that as well as players who've explicitly stated that they want to play flirtatious characters (though not in the same game, obviously).

And of course just be cognizant of how much detail you're going into; you don't want to be describing the details of a makeout scene, but "Jessica presses in for a long kiss" does the job and doesn't violate most people's boundaries in my experiences as long as they're okay with the general themes.

Never really had it go "weird", and honestly, though I've ended up in an IRL relationship with a player in one of my games on two separate occasions, the in-game romance doesn't seem to be related (neither of the people I ended up dating stood out in that regard).