Everyone on here will tell you a tux is no good for an outdoor daytime wedding. Also if you don't have tux already I would assume you do not attend many/any black tie events so do you really need to fill that hole in your wardrobe?
I just got married last week at a winery. Wore an unlined tropical weave wool suit to beat the heat and it worked beautifully. The only sweating I experienced was from nerves!
Also check out the wedding q & a thread for more info and inspiration if you haven't already.
That is super helpful, thanks. If our wedding starts at 3:30 and runs until midnight or so, is it an evening wedding or a daytime wedding for purposes of dress?
And to clarify further, I don't feel the need to wear anything formal like a tux. It's more like I think it'd be fun to. But I'm getting the sense that a tux wouldn't be super (a) appropriate for a daytime wedding, and (b) super appropriate for a backyard outdoor wedding. My fiancee thinks that a tux would suit her dress well, but is really leaving this entirely up to me. My main concern is that I'd like to feel like I'm wearing something special -- and given that I have business suits in the major colors already, I'm struggling a little as to how to make it feel special. Sadly, I'm not going to be able to pull of searsucker.