I would draw the line at "older" guys checking out my daughters.
That said...
I have never been mad or jealous of other guys checking out/flirting with my ex-girlfriends (going back to my early teenage years) or my wife (we've been married for 14 years, and together 16 years).
What made me mad was ex-girlfriends going behind my back and cheating.
With my wife, it's totally different. There's always been open and honest communication... So when she approached me about rekindling and intimate/romantic relationship with her ex-boyfriend from high school (they were high school sweethearts, when they dated for 3 years), I agreed and gave my blessing almost immediately. They have known each other since they were 12. I knew they had stayed in contact and remained "friends" throughout the years, and that he was someone very special to my wife. I know know a lot of people would see their spouce reconnecting with an ex as a red flag, or a no-no, but honestly, I have never felt my marriage was in jeopardy or threatened by her reigniting a relationship with him. He's actually a great guy, even though he's STILL a bit awkward around me, nearly 6 years later.
Ironically, their prior history together made it all that much easier for me to accept.
My wife's relationship with her boyfriend has NEVER been about "replacing me", and all about enhancing our already (especially hers) love life.