Have you ever seen a couple on a date. She's looking really high maintenance and he's looking like he's been sleeping for 8 years straight? She's wearing a nice fitting dress or a nice matching outfit, very sophisticated and he's wearing an old jersey from 10 years ago and his pants half way down his legs, exposing his boxers every time he walks? They just don't look right together. Their like bookends. They don't have to be the same size or color, as long as they look good together its fine. Thats a very bad analogy, but you get my point.
Suit with jeans? I don't think so.
It depends where the couple is going too. You're certainly not going to wear a suit to go bowling or to eat at McDonalds. If you must go there, go with style. They say when you go to church you should wear your best. I feel the same way about the girls I date. Doesn't have the be fancy but something really nice. You should see my closet. Shoes match shirts, shirts match hats, hats match with pants and pants that actually fit me and aren't 10 sizes too small in length. I have my fancy fake gold watch, my cheap cell phone hanging off my belt, fancy cheap sunglasses. If she's going high maintenance so am I.
I realize some guys hate dressing up, so what? It's for your girlfriend, if you can't do something so simple then why date? I always say if you can't make these little small sacrifices during the dating years, marriage will not go over so well so might as well stay single until you're mature enough to make sacrifices for people you love.
You don't want to embarrass her either. Iron the clothes, wear clean clothes, wear matching or color coordinated clothes, wear clean shoes and wear your pants up like a man. I swear I've seen more of guys underwear than I have of my own! Who started that style anyways? I remember a wrestler back in the 1990's who wore his pants like that and since then every guy has copycatted him. Anyways Gentlemen should dress appropriately. Don't worry about your "boyz" or your "rep" just worry about the girl on your arm. For the next few hours, she's the only one that matters.