Long time reader, I have read the entire sidebar, the PDF handbook, followed off-Reddit manosphere blogs, etc. Mateguarding has very conflicting advice here (set boundaries but don't give ultimatums) and I'm looking for some insights from like-minded men.

I just got into my first LTR since swallowing the pill. We fucked for 6 months before having 'the talk,' she's fun and the sex is great, yada yada yada. She's no unicorn but she's hot, fun, and I think the red flags are manageable or they have been up till now.

I just realized that this dude she's been going to yoga with (since prior to us hooking up) is someone she had a fling with last year (he dumped her). She probably started going to yoga with him so he'd see her getting all bendy in yoga pants and regret his decision to break it off. Obviously she's his backburner and it's not okay. As I see it, my options are:

1. Mateguard/ultimatum/BB shit (bad idea)
2. Pretend I don't give a fuck. I'm not that invested, and she believes I don't give a fuck, but obviously it actually does bug me otherwise why would I be posting here? This was my initial reaction to her telling me they used to bang, so this is how she currently perceives me.
3. Passively make fun of her for it. "How's the fallback guy?" "How was your yoga date?" etc.
4. Set hard boundaries and evaluate her response. "I don't date girls who date exes"
   1. Problem A: this seems like an ultimatum and shows I obviously care
   2. Problem B: she currently believes I'm so unthreatened by this and that IDGAF. This will shatter that image
5. Employ dread: tell her I'm going to get back in touch with some old FWB's. "When you told me you used to fuck <yogaguy>, I realized it's been foolish of me not to keep in touch with my fallback options too. This relationship will be healthier if we both have backup options and don't need to stay in it/are motivated to be on our best behavior. So I reached out to a few old FWB's this week just to touch base, and I'm meeting one for drinks next week"
   1. Problem: this doesn't explicitly communicate that her behavior was wrong
   2. Benefit: this girl gets jealous af and will freak out with a little dread, may motivate her to end yoga dates on her own accord
6. Demote and move on. Probably can't demote, would have to break it off completely. We have lots of mutual friends and it would be messy for a while but not the end of the world. I'll try one of the above options first (or something new that you suggest) and then evaluate her reaction. If she reacts poorly I'll break it off.