<human>: Condense the text into a few key points

I like to think my gf and I have a pretty stable relationship.  We have talked before about how cheating is unforgiveable and how if one of us met someone else we had feelings for, we would immediately tell the other person and break off our relationship.  A few days ago I saw her texting one of her attractive male friends and I could see above her response the guy had sent a photo.  I couldn't tell what the photo was of though, I just saw the bottom of it.  I didn't mention anything and I don't want to jump to conclusions and look like a psycho but I have been lied to and cheated on in the past so I also don't want to risk wasting my time in a relationship if she and some dude are exchanging photos of each other or flirting.  Is there any way I can say "If you're interested in someone else and I am second best, can you please just tell me so I don't continue to invest in our relationship?" Or should I just focus on reminding myself that this could be totally innocent and despite my past negative experiences not every girl is out to get me?
<bot>: is there a way to voice that you are not comfortable being someone's second choice without outright implying that you think they're up to something?