I read this whole thread. Not a lot of clear-cut advise so I'll be the one to tell you some actual what-to-do's, not sex advice, I won't tell you that your insecurities aren't real or that you're bad in bed or that she's big dick crazy or that she's loyal and wants to marry you because at this point you'll be dancing with the devil in terms of honesty. The only thing you can salvage from this is that she misses her ex because she's even thinking of him. I don't even think of my ex's or discuss them at all. I can almost gauarantee that itch will be scratched.

Here's what you fucking say to her.

You're my fiancé. I love you but I'm insecure right now because I saw your message about sex with your ex by accident. If I can't please you like that then why are you with me? If I'm not your best than I'm afraid you'll be wanting the best and you won't be getting it from me. I don't see where this is going anymore because even thinking of your ex, discussing sex with him is my #1 red flag if you're also going to be asking about what he's up to. If you don't say now exactly what I need to hear; we're through.

Ask "is it a fantasy". You need certain answers from her so that you can best be the judge of what is or will happen. Don't ask any other questions. If she does say "no, it's not a fantasy", than why is she thinking about it? That would give me the answer I needed. If she says "yeah, it's a fantasy", that would give me the answer I needed too (I'm not her fantasy). I'd run for the motherfucking hills with yes or no. Any answer other than profuse apologizing, a seriously open heart to heart and everything in between, I'd run. Because she isn't just being dishonest with you, she is being dishonest with herself.

Even if she was just shooting the shit with her friend, it was inexcusable and was very telling otherwise and she should have to deal with that, not you.

She's not just appreciating sex with her ex. She's complaining by comparing.

This is pretty fucking unsalvageable. Marry someone who can at least respect your dick.

Just because you've been with her for 3 years means fuck all. Loyalty means fuck all without a need. When someone no longer needs you their loyalty will fucking change. Are her needs being met? Obviously not, maybe. Not that you need to feel insecure about your dick, it's her needs and you have to respect that but it's the dishonesty that is the real problem. It's infidelity.

Now I see all these comments talking about either being better in bed or that maybe there's way more to sex than just dick size. There is, but no. No. And no. She isn't just fantasizing, she's asking what he's up to. Red. Fucking. Flag.

In 3 years you havnt asked her if your dick was big enough? You aren't ready for marriage.