Yes, I would refrain from such conduct even if there was no chance it would harm my love interest. I say so because I think the conduct is wrong, and also because I think the relationship is harmed (on my end) even if she never finds out.
My ability to be the type of romantic partner I want to be would be undermined by any illicit behavior like that.
As for the men in your life, honestly I did consider what you are saying and discounted it as fairly silly: I'm distinguishing from the instinct (which I do maintain is going to usually be present) and conduct (which is tempered).
I think you are acurately describing their conduct and articulated outlook, but are not offering much genuine insight into their instincts. As to why, I can't say.
The idea that the typical male would not instincively respond well to the prospect of sex with attractive, fit, beautiful, and willing young women is science fiction.
I don't doubt that most men temper their instincts, and ultimately become more domesticated to the idea that would rather have a more fulfilling and extended sexual relationship rather than a fleeting one, for all sorts of reasons.
That being said, the appeal of an uncomplicated, uncostly, unemotional, and uncluttered sexual adventure with a young, fit, attractive, goodlooking, leggy woman is very strong in most males.
If you don't agree, then I think the odds are higher that you are misreading the landscape, rather than I am misreading the landscape.
Trust me, I know that most men would jump at the chance. My point is that there are a few that would not--a few that find sex with a woman they love far more satisfying. But as far as being able to offer more insight into their instincts, alas, I am a 24 year old woman who has yet to understand the male species--and never will. What better a topic on V-Day?