Believe me, once this kind of situation starts, there is no going back. If he has had gay experiences and wants to repeat them he is probably bisexual. Are you ready for that? It does sound as if he has no intention of stopping for your sake and as if he is really into it all. There is nothing wrong with being gay, or bisexual, as long as both partners agree. You obviously do not.
Think long and hard about this one. This is not going to go away. Decide if this is something you can handle in your relationship, and if the answer is no, then you may have to move on.
In a successful relationship, both partners should agree on this kind of subject. Sex is a very intimate act, not to be taken lightly. Also consider the dangers of STD's and AIDS.
I truly doubt he will only ask you to watch him perform sex on another man. In the heat of the moment it will probably turn into penetration. If not, then it will in the future. Are you ready to witness that? I doubt it.
He should not expect you to see that if you don't agree... it's hard, I know, but are you willing to be with someone who has other sexual issues, different to yours?
Think long and hard and go with your gut feeling. Good luck!
http://www.healthboards.com/ubb/heart.gif Nela.