If my sexual orientation has truly changed, I'd already accept it. Otherwise it hasn't really changed since I have attachment to my "old" sexuality... But let's pretend that somehow my sexuality could change, and that wold bother me for some reason. I am presently bisexual (technically pansexual), so would the alternative be asexual? If then, I'd have no sexual interest... and that could hurt my relationship a lot. But I could also look at this from the angle that despite being pansexual, I am MORE attracted to men than women, so if we were to flip it around, I'd be more attracted to women than men... And the only reason this would bug me really is because I am in a happy relationship with a man. I would start to find myself wanting girls more, and that would hurt our relationship. The consequences of my sexuality changing would hurt my relationship a lot, but other than that, it doesn't really bug me.
I would be pretty worried about it changing so quickly, so I might deny it at first and question it. I'd let my partner know right away, and we'd talk through it... I would try so hard to make it work because I love him very much but sex and attraction is important to us both. Now personally I don't think sexuality is so black and white as it is in your example, but I have to pretend it is to answer your question. It would mean I no longer like hugging or kissing him, or any of that... And I would be really freaked out that my preferences had changed so quickly, for what would also seem like no reason.
I would tell just about everyone about this because I would want to understand why it happened and seek help.
If this happened to my partner, and he started wanting to date girls, I would be really freaked out and upset, and it would almost certainly end our relationship quickly.
Just Another Face -GM-
I legitimately have no idea. Because I am like... 99% straight with a niggling curiosity what cocksucking is like. So I guess if it happened, curiosity solved.
Can you reach your own d**k with your mouth? I've had the pleasure of seeing my own man be able to do this. >.> It helps that he has a big c**k AND is hella flexible.