I think the answer is twofold: 1) Guys who would fuck another man are not straight. Maybe they're straight-leaning bi, at best. Perhaps a straight guy might be able to receive head. But hell, even I can't get up for just any guy and I've been topping for close to 50 years; 2) It's infinitely easier to find a gay guy to give them what they want, especially if they're decent looking and don't require a certain "type". It's unlikely you'd find a random woman online willing to lay there for a pump and dump, unless you're willing to pay.