<human>: Both mid 20s, relationship for about 3 years. Cheated on me 2 weeks into the relationship, then fell in love with and secretly tried to get with a musician friend of hers for a year, we're somehow still together.

She's recently been trying to get back into playing guitar and has a music teacher, someone she met while away at college (so she says). According to her he's married, but she mentioned he's thinking of getting divorced.

Her first lesson was today. She asked for $25 (a small amount for an hour long session) for the session, and seemed surprised when I offered her a ride (she was just going to take the bus). I figured it was just like a four minute drive so it was the least I could do. Turns out they were meeting on a bench in a public park. I thought that was a bit strange.

I was a bit suspicious of this music teacher, just because I know she has a thing for older guys and musicians, so I decided to circle the block just so I could have some idea what he looked like. Things seemed totally fine, so I headed back home. I started getting an odd feeling, so I turned back and went back to the park. I saw the music teacher had his hand on my girlfriend's shoulder, which seemed REALLY sketchy. 

I drove off so I could cool down and be rational about things.

I decided to pick her up. I just so happen to get there early and see they're much closer together on the bench, and instead of practicing they were talking and staring out at the park. I pull up and when they notice me they go back to looking through books.

I asked my girlfriend what was up and she got defensive, saying he had never touched her and that they were just talking about a song. I don't know what to believe, I have been feeling more paranoid of late, and my girlfriend did work really, really late for the last couple of nights (last night she didn't get home until 10) so maybe I'm just imagining things.

Condense the text into a few key points
<bot>: GF has cheated before, is seeing a sketchy guy for music lessons, and claims he never touched her when I had seen him doing so. Should I give her the benefit of the doubt, probe deeper, or what?