In a word, yes. For one, is this guy your best friend? If so, why not try to find a way to get your groom to let him be a groomsman?
While I admit that it's a nice gesture and shows how close you are and how much you appreciate and respect this chap's place in your life, the fact is he's a dude and it will not only look weird, it will be weird. I have a lot of female friends who I'm probably close enough with to warrant being a "bridesmaid". However, I'd much rather just watch the holy union from the rafters than stand up there as the odd man on the woman's side. I'm not sure if it's a pride thing or what, but there'd be something uncomfortable about it for me. Despite nobody openly saying anything, I'd feel like my masculinity was being questioned. I'd probably have to show a little chest hair just to make myself feel better.
Additionally, how does that work with walking down the aisle? Does he go down by himself? It seems like a logistical nightmare as well. I guess the bottom line is this, if you feel that strongly, you should ask him. If you're husband is okay with it, and your buddy is okay with it, then great. It's your wedding and you can cry if you want to. However, I'd be prepared to hear "you know, I appreciate the thought, but I think I'm just going to sit with the family on this go 'round and try to holler at your sister who wishes she was getting married."
It can actually totally work - if your fiance has a sister or female friend he also wants to have stand up for him. When my brother got married, his brides two brothers were her "bridesmen" and my brother's female friend and I were his "groomsmaids." All the female attendants wore the same dress and all the male attendants wore tuxes, but we stood on the side with the half of the bridal couple we were closest too, and it didn't look weird at all. It's all about reciprocation - but really, isn't it always? :-)
I actually did this in my wedding and it was awesome. My husband had two friends who were girls (they were sisters) that I didn't know all that well when we got married, but he really wanted them to be in the wedding. They wore the same dresses as my bridesmaids, but stood on his side in the wedding and my two brothers wore tuxes and stood on my side. It was really cool because in the wedding party, those are supposed to be people who you are close to and who are there for support. Some of our family members who were way too concerned with appearances thought it was weird, but we kind of said "who cares what you think?This is important to us." and did it. And you know, everyone thought it was really cool and we were happy.
When my friend Chris married his girlfriend, I was his best man.
I'm certifiably a short little blonde chick too.
We did it not only because we're tight friends, but what also had a lot to do with it was the fact that the rest of his guy friends were irresponsible sloppy stoners and d-bags. Any one of them would have lost the rings on a munchies run.
My best friend's husband had his sister stand on his side at their wedding. They are very close and he wanted her up there with him. Besides, isn't that what it's really about? Having those closest to you up there supporting you on your big day? Who cares about gender? Nobody seemed to have a problem with it, but who really cares if they did.