Dear Hootsweet ~
I have no instant answer to your problem. You are very obvious a caring person wiht a sensor of responsibility and the ability to love. it is unfortunate to say the least you are in a relationship that could easily become permanent that may go against your sexual identity.
May I offer some thoughts?
I admit they are probably biased as I'm old enough to remember people getting married due to pregnancy as a duty -irrespective of their feelings for each other, society demanded it, and things did not always turn out well.
Say you stayed with him, not wanting him to be hurt and tried your best to give him what he needs -at your expense.
Do you think things will remain the same between you or might either of you build up anger and resentment later on at a realtionship based upon sacrifice?
Again in the future a desire to "wander" might be very powerful.
On another matter in a way at the moment you live in a protected world, with someone to look after you, love you and be prepared to live their life with you. You in turn react to this and try to make that person happy, look after them.
'Looking after' is both short and long term.
Should you leave you may find that you are not as fortunate, and that while sexual partners may be available, the nurture you are used to is missing, a different ball-game. Plus from the sound of it you may not have had a great deal of experience of queer sex and relationships, forgive me if I misunderstand.
If I look as myself (I'm straight BTW though I'm not sure that comes into it) and wanted to be with someone long-term I'd have to know myself well enough to make a sincere offer of partnership, and that includes my sexual identity. I would have no plans in mind to try other things later.
I suppose I'm saying in my own case I'd need to find out who I was and then choose or be chosen as a partner on that basis.
Please do not think I'm trying to steer you in any particular direction, I do not have a religion and think love, honesty and cherishing are the most important elements of a relationship ,and that includes the genuine intimacy and pleasure of sex, an important bridge for both people.
I'm not sure I've helped much, talk here as much as you would like, I'm sure there are many who will understand exactly.