Most men don't have 12" cocks and fuck everything that smiles at them, either, including their moms, sisters, daughters, cousins and grandmas (well, unless they're royalty or from Appalachia, though the 12" cock is unlikely in those scenarios), but at least a few of those elements seem to be common to nearly all porn and the majority of AVNs. And don't even get me started on Japanese shit with tentacles, vore and everyone making an ahegao face from the first touch.
Les/Bi is just a common trope and a guy with two or more women (friends, IDENTICAL TWINS, sisters in general) is one of the more common fantasies out there. People trying to make money at this are, in large part, playing to the largest part of the demographic that might send them coins, not catering to the niche folks (unless that's their kink and their only focus) and those with seemingly Puritanical "fuck the women, but don't let the women touch each other - and, BTW, it's also okay to be a little cruel to the women since, after all, men are superior in every way" crowd. Unless you're a SJW, marketing 101 is play to the biggest audience. The overwhelming majority of VNs will do this.