Sounds rather normal tbh, I have good looking, rather attractive, younger wife (not the super model type but it doesn't matter to me, she has great character, she is smart and is very into me) - I'm not model-looking level either - so we are matched well. I still perceive other females attractive and I understand that by deciding to be with somebody I also decided to not be with other women - this artificia limitation makes your internal "hunter" triggered from time to time as to some extent we are animals and we want to mate with as many women as possible - the novelty of relationship and interest in sex also wears off to some extent and relationship moves more into friendship zone from lust zone after few years. The thing is to throw in some fuel to raise the flames from time to time (it is also ours task to do it as we should lead in the relationship). This is more difficult to do than to think or plan about it (because ordinary or extraordinary life issues messes up our initiative always) but that's how the things are.
So I think its normal and common - some guys act on these urges and cheat. Some don't (they turn the sublimation button and decide to use the spikes in energy either to make current relationship more comfortable or just concentrate on themselves, their hobbies, work etc. and decide (or perceive - after proper or faulty assessment of their current situation) that there's nothing that can make this situation better/different and just carry on.
Biking doesn't work miracles when it comes to stomach fat. I have a buddy that does 200-300 miles on bike few times a year and rides shorter distances often - he also likes beer and beer wins with all the biking he is doing - from what I have noticed, exercices that make your stomach do something else than just being there (e.g. sideway turns, crunches, also going low-carbs diet) are the only things that may help a guy maintain his belly in check.