That's a large part of why I avoid 1x1 RPs, tons of them are just forced romance.
But yeah, it is rather annoying. Some people are indeed obsessed with romance and won't give the idea up even in a post-apocalyptic zombie survival RP (for example). It's not a bad thing per se, but it can be very irksome to have to deal with such a person if you don't like the forced romance thing. Forced romance sounds like rape, but w/e, the term fits.
Anyway, if it's a thing in 1x1 RPs you can head this off at the interest check phase. Just ask the person what kind of RP they're looking for. Those seeking romance stuff will very likely make that obvious and you can thus go another way. Or avoid any interest checks that list pairings the person is interested in RPing, since that seems to very often be romantic pairings. If you're making your own interest check you can put a note in there somewhere that you're not interested in random romance just for the sake of it, so you'd prefer people not come into the RP seeking such things. I think those couple layers of precaution should be enough to weed out the forced romance people.
Groups are a little more tricky since you're dealing with multiple people and can't really weed out romance types unless you're GMing it and do some kind of strict interview application process. The best way to deal with it in group RPs is, when you see it starting, tell the person that you're not interested or that your character simply would not feel that way because of whatever reasons. OOC thread, VM, PM, take your pick. If you get it early enough they'll either just back off or switch their silliness to another character. If that does not work, have your character continue to act in whatever way they would to the advances while making it clear IC that it's not going to happen. Should there be some kind of OOC harassment problem with it you can ask the GM to step in or send in a report for a mod to deal with it should you feel it's enough of a problem to do so.
Hopefully that helps with whatever your current irritation is that drove you to make this thread, or with future forced romance issues at least.