[A shrug.] To keep you busy. [Obviously a good reason. He's not explaining the bachelor party thing though, he considers that self-explanatory. If Luffy gets to be his best man, Marco gets to throw the bachelor party. All when Ace marries an as-yet unknown person :|]

And so I'll have something great to tease you about. And then you can have babies and I'll play with them. [He says... as if Marco would ever be allowed near the kids.]