You know it's not a bad thing to be different from your partner's other love(s). It just shows that he loves things about you that he can't get anywhere else just likes he loves things about her that he doesn't get anywhere else.
My husband's girlfriend and I are quite different creatures. In a lot of ways she's quite like him interest wise. They are both the more stay at home and do something quiet sort. I think it's awesome that he's found someone who genuinely shares those interests rather than me who's happy to play along (rpg's and the like) but could really take it or leave it.
Shared interests are really just a surface thing anyway. Personalities that are compatible don't always have a lot to do with shared interests. I'm pretty sure that after 5 years of marriage both of you just know that you get along (even if it doesn't make much sense on paper).
Why would he want to chose? If he can have you both and fulfil both aspects of who he is in those different relationships. Give it some time and the shiny newness of it all will wear off and then reality will set in. Relationships go through cycles, there will be times when he will crave your company and times where he will crave hers. It doesn't make either relationship better or worse, just different.
Everything will be ok in the end. If it's not ok it's not the end.