His daugter has asked me to sit with her Mom's boyfriend who I have never met. He won't have other family members there. Am I out of line is I say I prefer to be with my kids and family and suggest the boyfriend sits with the mom's family?

I don't think that's out of line at all. The boyfriend isn't a part of your family, he's a part of her mothers family. It makes sense for him to sit with them, not you.

I think your boyfriends daughter just wants to see her parents side by side on her wedding day. I know that's what I would want and yeah my parents are devorced

I think it's the daughter's wedding and if this makes her happy (perhaps she's concerned that her mother's boyfriend will be alone) I'd do it.

At your wedding you pretty much get to do whatever you want. That is not the case when it's someone else's wedding.

I don't think who sits where during the ceremony is a big deal.

While I see it being awkward, I agree that the daughter probably wants to see her parents sit together for her big day. There isn't much time to socialize during a ceremony anyway, so I would stay quite for the sake of keeping the peace with your step-daughter and honoring her wishes.