I'm a bit older and in a monogamous LTR. I don't feel any desire to cheat. I suspect that partly I'm just not wired that way, personality-wise.
We both came to the relationship with a bit of experience under our belts, sexually. It's not like I could write a modern Sins of the Cities tell-all about my escapades, but, you know, I explored the buffet of available sexual experiences to my liking. Enough that I don't have to be wondering if there is some transcendent sexual experience waiting for me out there.
Also, our sex life is satisfying. Offhand I'd guess that my husband and I have had sex over a thousand times. That's just arithmetic, not boasting. We've gotten pretty darn good at working each other over. It's hard to imagine bonking a stranger would be anything but disappointing. I find my husband really hot; we stay fit and go to the gym regularly, and part of the reason I go (in addition to health/fitness) is because I love that I turn him on and want to keep it that way.
Sure I see guys at the gym or on the street who make me think "dayum," but it doesn't really compare to looking at my husband and thinking "He is so beautiful and I love him so much." Also, I think one of the under-appreciated benefits of porn is that it desensitizes you to extremes of male beauty. If I see someone really hot, I appreciate it, but it's not like when I was in high school and would be transfixed at the sight of some guy because he was the hottest thing I had ever seen.
I'm old enough now that my sex drive isn't as urgent; when I was younger I occasionally did dumb shit because I was being led around by my cock, but these days I'm more in control of myself. I have a certain sense of self that I want to maintain; I know that if I cheated I'd be disappointed in myself and it would haunt me.
I don't really go out of my way to avoid situations that could lead to infidelity. The lives we lead don't expose us to much temptation, though. We're both busy professionals with demanding jobs. We're not the type to go to dance clubs, I don't think I've been to a gay bar in the last decade. Our idea of a fun night is cooking a yummy meal, having some drinks, knocking boots, watching a movie, and snuggling up in bed together to read. I travel somewhat for business, but the guys I work with are mostly middle-aged nerds like myself and I only know of one other out gay guy at my company. I'm shy enough that if someone was cruising me I'd probably just blush and study my feet until I thought of an excuse to run away.