The men in it would usually be the ones pleasuring the girls as opposed to them getting it - which will be awkward and sexually frustrating for a gay man. They would still do penetration, but wouldn't cum quickly, and perhaps, trivially, pleasuring the women more.
Gay men doing straight for pay get a lot of money for doing this, either way.
Recently outed lesbian Tila Tequila has had porn with men. She admitted that it was straight for pay.
Person 2: "Wtf? They're getting married? But Jon is super gay and Gina is a diaper dyke...how is that possible"
Person 1: "No bro its just straight for pay. They have to do it because the government and Christians say that they can't marry who they really love, but they need the benefits of marriage"
Person 2: "That fucking sucks dude. That's so breeder. Fucking Hetero bitches"