Boyfriend pretty much means insurance for women. Her hypergamy will continue to search for a better prospect to emotionally conquer while she has you as a motivational trophy. A women's main goal is to tame an alpha. Once that alpha is tamed her attraction will faulter. Subconciously she will think, "Why are you only fucking me? You should be fucking other women. I have you secured for now but you're slowly (or quickly) becoming less attractive."
Fuck other women if you want to and never lie or feel guilty about it or you will come off as beta. If you don't fuck other women you're subconciously becoming beta in her eyes anyway. If she leaves you she probably found a better prospect or you weren't that special. Remember, women would rather share a 20% alpha before fucking the 80% beta. A tamed boyfriend is a cute word for upgraded orbiter with benefits.