I think your instincts are right on the mark!  If he accepts the possibility, then I believe he is more susceptible to doing it.

It is obvious that he has already been thinking about it, since he brought it up in the first place, and is now setting the stage for forgiveness "in case" it happens. It sounds as though he is already justifying it in his mind too, by implying that being drunk and losing control of yourself is an acceptable excuse. Well, it is not. (I, for one, would never put up with someone who even expects to get drunk and lose control in the first place. You invite too many opportunities for things like this -- infidelity -- to happen. I think he needs to grow up and accept the responsibility of being a monogamous relationship.

The question is, how do you handle it? Disappearing without warning may not be the answer. You may want to tell him how you feel about this, and then set firm standards for what you will and will not accept. If he violates those standards, then you need to be firm in your reaction by saying goodbye. If you waffle on this, he will not take your feelings seriously, but will know that he can get away with hurting you.

You said that his friends are a bad influence on him. Do you see his behavior being affected by his friends? Has he proven to be easily influenced by them? That may be a good indication of his character. You are smart to notice that, as immature guys can be dragged down very easily by the wrong crowd. If this relationship has marriage potential at all, you need to be very observant of his character and his behavior patterns.

One more thing, I have often told people at Love and Learn that according to the Bible, pre-marital sex is wrong. God has good reasons for why He commands things, and I think we can all avoid a lot of trouble by listening to Him. After all, He made us, so He obviously knows what will fulfill us.

One of the many reasons He commands us to stay pure until marriage is so  that our marriages would be completely exclusive sexually. No "baggage" from past relationships, no pain and disappointment from unfaithfulness... He wants us to be completely pure and given wholly to our mates, to share the most special of moments on our wedding night, unspoiled. I think that is what you want in this relationship -- complete monogamy -- but God goes a step further and declares that this is what He wants for your marriage; for you to share physical intimacy with your husband only. Some food for thought...