Hiya JoJo -
welcome aboard. Although I myself didn't/don't have POCD, I know from others' writings here how taxing it can be, esp. in the form that you describe here. I hope that a few of these members will chime in and comment on your post; and most likely they can refer you to more stories from people who share your brand of POCD; perhaps they also can advise you on other matters, e.g. a book about this affliction.
More generally speaking: you yourself can't see that it's just OCD anymore. I myself think it is; and I think that it is so frightening and unsettling for you, because it feels like a threat to the very core of your personal happiness/existence, i.e. the bond you have with your boyfriend. That is how mean OCD can be. It can make use of one's tendency for worrying and being afraid, it feeds from that energy, and then it takes on the mantle of something evil, something that makes you doubt your sexual identity, and thus the intimate friendship you have with your boyfriend, so far as to make you feel like a liar and a cheater.
You aren't.
OCD is playing up terribly; and makes you doubt your own personal identity, and the nature of your love for someone. I'd almost say: don't let it fool you (but that may sound too flippant - or does it?).
At one point you say: 'at the moment'. Somewhat later you ask: 'can your sexual orientation change'? I am inclined to say: one's sexual orientation won't change. There are a few cases I've personally heard of where someone, after a struggle, can admit that she/he is in fact bisexual, instead of hetero- or homosexual; and it takes a long time to arrive there, not 'at the moment'. But a radical changeover, that I've never heard of.
Did you talk to a psychologist about this specific fear? If not, you need to, I think. And: do you use medication? If not: it may help a lot in suppressing intrusive fears like yours. Could be that an added neuroleptic, e.g. quetiapine, makes for an even greater beneficial effect.
That's what I can come up with now. Mind: I am not a doctor, nor a psychologist; I'm someone with OCD myself (checking and hoarding); and a trained neurobiologist.
Please do keep us updated about things, and always feel free to send me a private message. Wishing you all the very, very best, kind regards,
Cuthbert in the Netherlands.