This is my opinion, mind you, I'm not suggesting anyone has to agree with it.
I personally think it's extremely bizarre that there are mandatory vows of celibacy at all in a religion in which the god falls in love with and tries to coax away (seduce?) a married woman. Yes, good people make mistakes. But if you actually believe in those oaths, you're going to be so miserable once you think about what you did, you'd never enjoy it (unless you just enjoy guilt itself). But if my character was actually under vows and cheated on them, I can't see how she wouldn't be drowning in guilt until she stopped. If you believe those vows mean something, you're going to try to live within them. If you took vows you don't believe in, you're at least morally/ethically grey. I suppose there's also the possibility of just asking to be released, but I got the impression from the Sebastian story that it wasn't nearly that simple (i.e. he might be able to ask to be released because of his duty, but not just because he wanted to have fun).
Marriage is the perfect analogy (since that is essentially what that vow is). Unless you have a defined open marriage, if you have a fling with someone, you're cheating on that vow. Yes, good people can make a mistake, but unless you're at least morally grey, you're going to feel very guilty about it once you have time for it to hit you. And unless you have no respect for the marriage vow (grey), you're going to stop or get a divorce so you can continue that relationship without the dishonor/guilt. I have absolutely no issue with someone in an open marriage doing whatever they want without considering that grey; they aren't violating a vow (and I do in fact have friends irl in such relationships). When I've kept a Warden as Alistair's mistress, it's always been with Anora knowing and agreeing. One of my worldstates is going to have a HN in a state marriage with Alistair, but romancing Zevran and all three knowing that's how things are (and she'd be perfectly happy if it became an active polyamory) but it's an honest, guilt-free relationship. Me, I can do all sorts of things to my PC, but drowning in guilt connected to a romance? Nope. Guilt of that sort has no place in a healthy romance (or even a healthy fling). Whether it's just sex or romance, it should be enjoyed without a freightload of guilt.
Dang, that got long-winded. What this comes down to is if a Trev romance requires a crap load of guilt to go along with it because you're violating a vow, I'll pass. There are too many guilt free options.