I can't claim to be an expert on straight porn, but from what I've seen, the guys are "totally solid" in straight scenes just as frequently as the tops are totally solid in gay scenes... that is, more often with high-budget production and not so much in cheapies. Since women don't have erections, per se (or certainly not as noticeably as a man), you can't tell whether or not they are "enjoying" the act as much as one would in real life, so comparing a woman to a bottom in porn is an issue of apples and oranges. And then there's the whole heterosexuals-acting-in-gay-porn issue, which I'm sure has a somewhat chastening effect on the performers.
But like I said before, you cannot judge real sexual situations by what you see in porn. Porn is make-believe, staged and artificial... it looks like sex, it makes you think about sex, it's a representation of a fantasy of sex... but it ain't sex!
In real sexual situations, you can be having a real hootenanny of a good time and still be pretty much limp (getting fucked feels really good in your ass, not your dick; and though your dick can be aroused by the activity, it's not necessary to the activity if, you see what I mean), just as you can be hard as a rock and not really enjoying yourself that much (it's very easy to give a man an erection despite his mental state, if you know how; with the proper stimulation, applied or accidental, you can keep a guy hard while torturing him or even just boring him to tears).
I've read so often about people wondering what's wrong with them that their sexual activity is different from what they see in porn: why isn't my dick that big, why don't I shoot that much, why do I get shit on my dick when I fuck someone's ass? Or else I read about people wondering why anybody would want to participate in those sexual activities because they don't look like much fun.
And the answer is that porn is not real. As soon as you find yourself having it on with another guy, especially for repeat performances, getting to know him, sharing more than just your body with him, these things become clearer. And talking to people in person or on message boards gives you a lot of this information.
But don't look for truth in a shiny package retailing for 69.99 at your local Ye Olde Adulte Shoppe.