No, if they have a guy/girl already it would be so fucking wrong to bud in... If they really want you they'd just break up with the person, I mean I'd dive in, but it's wrong
i was ina situation like this a while ago. I met this guy two weeks after my ex boyfriend and i broke up and i was feeling a little low and this guy and i had a little kiss that night and we would meet the following week...anyway, turns out this guy goes to me and says i cant lie to you, i have a girlfriend and we've been together for 5 years and we're to marry soon...so i'm ok, so what do you want me to do? at that moment i didnt give a fuck and i just wanted his company really. I wasnt planning to steal this guy from his girlfriend, because after all, i was still hurt and feeling weird about relationships. I distanced myself from him but he kept texting me and calling me saying stuff like i want you, blah blah blah..even until a few months back, he texted me saying he wanted to meet, but i got a little suspicious of the way he texted, i presume it must have been his girlfriend who probably found my number on his mobile and try to trick him or something...i just replied saying, please stop texting me cos i'm not interested and i'm sure your girlfriend wont appreciate you meeting another girl somewhere...he never texted again, and i'm so glad. thinking about it, he was sort of creepy... *shudders*
A few years ago, I might have said 'you're not the one with the obligation', but now that I've seen it from the other side, that's just not cool.