Hi 82Milly, this must be difficult due to you having kids together, whatever the outcome...
I don't know you or your partner obviously, but sharing a bit of my story might help to fill in a blank or two; that's the premise anyway.
Around 14, I started questioning my sexuality and sexual preferences. It was a confusing time, due to having issues at home with my folks and siblings. Let alone the social stigma of a species still in the dark ages on this front. Things are progressing slowly, there's this at least.
Everyone's unique, so I'm not sure if what I'm about to share will be of any assistance, but for myself I guess the truth is, that although I am at the point of looking to enter a serious relationship with the opposite sex (I'm M), one that hopefully we can create a family with but if not, to just be together as life partners.
If I happened to meet a very special man, I'd probably just as likely fall for that person. Irrespective of their sex. I'm a hopeless romantic. To be fair, I'm probably not your average mid 30's Aussie either.
What my years have taught me thus far, is I've actually the capacity to fall in love with a soul over a sex. This discovery only came to light after digging up this repressed stuff... and working through my feelings around this once hard to accept fact. Some early experimenting I guess (not all the way), taught me that love really is in the eye of the beholder. There are men (and women), who can relatively effortlessly look past genitalia, whether if seeking some fun, or a life partner. I prefer women, but a beautiful soul is a beautiful soul in my eyes.
This is a bit of a stab in the dark, but is there a chance that your partner might actually have an addiction to sex? Does he look at porn a lot, even if it's with you? Personal questions, asked only to possibly assist you to find some clarity in this. Actually, please don't answer as they were only intended to stir thoughts for you that you might have not considered.
If this isn't the case then I guess you've got two options here; take him on his word and if you ever find out otherwise, just know that this is about him, not you. Sexual preference can even be fuild and changing. We're a complicated species.
Now serial cheating; if it were me, I'd have to move forward without my partner, even if kids were involved. Our sanity and esteem is our most priceless asset. No one deserves to have theirs taken by a confused soul or worse, a covert narcassist etc.
Best of luck.