Playmate: The first thing you need to do is put the plans to move in together on hold until you and he have worked through this, one way or the other. One think you don't say, and I understand you may not know, is whether alcohol or drugs were involved in the sexual interaction with the other guy.
I understand the reason for your concern. It sounds like what you're wondering is whether he is actually gay. Many gay men try to live "straight" lives, and many marry and have children. In my counseling practices, I have seen several couples where the husband, usually in middle age, declares he has been gay all along and can't continue to pretend he is straight.
It is possible for a sexual encounter to be a one-time thing, homosexual or otherwise. If I were in your shoes, I think I would be suspecting there is more going on here. Ultimately, this is a conversation that only the two of you can have. What you don't want is to continue as a couple only to realize that he is gay and miserable trying to deny that. I think that's the conversation you and he need to have.
You need to prepare yourself for this conversation by being calm and prepared to hear whatever he has to say. One thing about men is that we're direct creatures. A lot of guys will hide things, but will give an honest answer if asked directly. Things you may want to consider asking is whether he has fantasies about guys and whether this has happened before.
One thing I would advise you to do is avoid a confrontational approach. Rather, I would recommend taking a "Is proceeding toward being a committed couple the best thing for both of us?" approach. If he is in fact predominately attracted to guys, then the relationship will bring nothing but unhappiness for both of you. It's better to figure that out now than fifteen years down the road. I see too many people in my practice who are going through that situation, almost always in their forties.