Jakual: Sexual compatibility is a very important aspect of any relationship. This is obviously very important to him. It sounds like you haven't tried it but are at least curious. What is the harm in exploring this and seeing where your level of comfort is once you've experienced it? Like all things, people have different comfort levels with different activities.
I think you're making a wise decision not to try to force him to deny this part of his sexuality. However, his desires in this area may be something that you just can't share with him. If that turns out to be the case, then that's a conversation you and he will have to have. On the other side, you could realize that that kind of play is very compatible with who you are.
If, at the end of the day, you decide that this is something you and he just can't share, that will be a problem down the road. Sexual incompatibility is a major source of relationship problems, especially 5-10 years in. There are a lot of possiblities. You could decide that you're comfortable with him expressing himself sexually in this way with other people. He could decide that a relationship with you is worth giving up this behavior. You could also decide that it just won't work out. This is an area where hiding how you feel will just result in much worse problems further down the road. You and he need to make sure that you're having very honest and up-front communication about this subject. The nightmare situation here would be for the two of you to marry and then realize that this is an issue you can't resolve. The time to resolve it is now.