I dont think I could ever be top

[marshlander]

Its not the putting my cock into another guys ass thats putting me off, I just don't think I can do it.

Its the selfish side which is worry me..

As Shadow says, there is no law that says you must. If your partner is happy with the arrangement, what's the problem? The important thing is to talk about it and be honest with each other. You could be surprised at what you might eventually want to do for someone you really care about.