Baby I Love You by the Ramones should work good acoustically with or without singing. Though not a punk song , who cares, it's by the Ramones.
The second "You may kiss the bride" was anounced at our wedding, "Time for Heroes" by The Libertines started, loud. The lyrics don't really make sense, but it works.
"Yeah, I pissed myself, but that's only because I only had 20 minutes for recess." Duane Peters
MxPx - Do Your Feet Hurt
I've got a bunch of high end vintage audio gear I'm clearing out, restored Marantz 4270 - better than new condition; Pioneer SX-5590, 8/10; Concept 16.5, 8/10 cosmetically - 10/10 function. Various high end vintage speakers: Rectilinear Research, AR, Kenwood, Infinity, McIntosh, Leslie 251. PM me for details, clearing out to finance home theater upgrade.
As far as a milder punk song goes, if your girlfriend is a punk rock girl, why not go with "Punk Rock Girl" by the Dead Milkmen?
The lyrics to that song are actually pretty fun and the reason why I like it. I typically listen to heavier punk (Strung Out, Bad Religion, etc.) but their songs wouldn't be as appropriate. You need something a bit on the lighter side...