As hard as it would be I would just ignore it and not give it a second thought. If she see's that you could care less and are more concerned with important things but keeps shoving this crap in your face then say "hey if you like this guy go have fun but let me know so I can make other plans tonight"
Her jaw will drop as you say this without a care in the world
Yep - that's what to do.
I see no reason to get mad at your girl - she didn't do anything wrong.
Would you like it if she gave you crap when your ex phones you and says something?
Sounds like you just gave us the reason for her to consider not telling you - your first thoughts seem to go towards stupid physical violence. Can't blame her for fearing your retaliation or future drama that will ensue if you did do something. Boyfriend with assault charges....you know her friends will be telling her to drop you ASAP before you turn to her. They will fill her head of stories of guys they used to think were nice until...but never just never saw the early warning signs. Don't give her those signs. Girls think differently than guys.
Besides, she only has 2 choices: tell you OR don't tell you. I'm going to guess people will find a problem with her choice no matter which one she chooses. Nice situation to be in...
Tells you - "oh, she's just trying to make you jealous and cause drama"
Not tells you - "lying biatch is keeping secrets from me"
As for her wanting to feel sexy - no big deal. Everyone at points in their life like to be reminded that they "still got it". It's human nature. Flirting and getting female attention is the REASON why I never needed to cheat. Making a big deal out of it is only going to hurt matters. Pretending you're totally supportive of it is usually the wiser choice.
Personally, I much prefer a girl that tells me I look hot in a sleeveless shirt and loves making all the girls jealous of her. If she tells me to not wear it because I look like a slut...well...it's only going downhill from there. Nobody likes a control freak.
You're 18. Statistics are very, very much against you and her lasting for the rest of your life. I'm not saying it can't happen, but chances are pretty good you're going to be with lots more girls after her. Spend less energy worrying about this other guy, and spend more energy networking situations that make hooking up with another girl favorable. You program them like sleeper agents that get activated if you're single.
You're not cheating - just preparing for the future.