I disagree with the "normal and natural" piece of advice. I don't think having gay sex is normal or natural for you (but, although it's not normal for you, you can make it look more natural). You're straight and it shows and that's a good part of what the sites about.
But, you want to be a good porn model. As Tampa said, you have all the right attributes going for you. At this point, you've done several scenes and I assume you've gotten (mostly) used to having sex with men. Most gay for pay models admit that after getting past the major barriers of sucking a dick or fucking a hairy ass or getting their ass fucked, they find some of what they do pleasurable. One of the all time great
Broke Straight Boys models, Jason, who worked here and at other sites and had (may still be having) a long career said that the more sex he had with guys, the more he liked the sex. He was able to find pleasure in it and eventually he was a passionate performer who's work was totally believable. It's guys like him that do really well in the business. Many other sites have a high percentage of straight guys doing gay porn, but often they are perceived as gay because they make the sex look so believable. On this site, a part of your straightness has to come through. I don't think you'll have a problem with that.
That's not to say that you don't also have to act. Some of it will never come naturally to you. If a scene calls for something you're not really happy to be doing, get your head wrapped around whatever it is BEFORE you have to do it. Scrunched up eyes and pulling back show an aversion to whatever you're doing and it kills the flow of the scene. Don't lick a pit or suck some toes for 3 seconds. It looks fake (which for you it is, but that's the acting part) and again, kills the flow of the scene. If you're going to do something out of the usual realm of "3 positions then cum", do it for at least 15 to 30 seconds. If you eat ass, you have to do it longer than that. If you take a load in your mouth, don't gag (but do close your eyes. Cum in the eyes burns like hell).
You may want to watch some of the porn on other sites, but I'd be careful about that. I'd say about 80% of the porn out there ranges from just boring to really terrible to really terrifying! Even on the good sites you're lucky to get one hot scene for every 5 mediocre ones. I don't belong to
College Dudes, but many of the members on this site do and really like it. It's part of BluMedia, but I think it's more of a "full tilt" gay site. The guys do more and make it look real. Talk to some of the other models or ask Johnny or Sha about other sites. Maybe they don't want you getting to look "too professional", since
Broke Straight Boys is really considered an amateur site.
At any rate, watch some of Jason's scenes. He always brought intelligence, humor and passion to his work.
He made the sex look hot. That's the name of the game.
Good luck, and I hope you can grow into the model you want to be.