Safety is a good thing. Boredom is a bad thing. For me, letting my woman feel safe with me and around me and simply being myself works and worked in the past wonders.
I let hew know that she's mine and I am not targeting other targets, however all my past/current girlfriends were all jealous, not me. Yes there's a reason to be jealous, as I attract girls without doing anything.
I may sometimes be boring or so, sometimes crazy and unpredictable and really, I can't sometimes predict what I will do 5 minutes later as I like to do what I feel like. For me it worked in a few year LTR and works in a current LTR of almost a year.
Now for the cheating thing, I believe it simply states that not all the women are bitches or assholes, but they are passive human beings or losers. However I consider not calling cheating girl a loser. I call them passive and not strong persons enough to leave the current boyfriend. However I always consider the boyfriend fault in case of cheating. A sharp guy can smell when his women is bored, lost passion or didn't even have one. This must be a good indicator that she will fuck someone else, especially when he is dark and the one

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In my phylosophy - if you're not 100% of a man for your woman in all areas: emotional, sexual, if you see that she doesn't look like the happiest girl on earth - it's your fault. If you don't make her laught, look with big eyes at you - your fault. And it works vice-versa - she looks happy, satisfied, uplifted - it's your pride.
Now back to the question - I wouldn't feel bad for fucking a girl in a relationship because if she does that, then her boyfriend really isn't "the one" and doesn cause the spark in her like you cause.