The funny thing... there actually is Gundam porn, and it is not about the pilots making out, but the damn robots. Rule 34 applies to basically everything. And of course, you can always count the fans to make the most unbelievable relationships happens.
I try to have an open mind, and as long as their fantasies doesn't hurt anyone or are actually illegal, I find it funny most of time. Kirk with Spock, Batman and Superman, Cloud and Yuffie. If you search for it on the fan fiction sites, you will find any kind of porn/erotic tale involving basically all characters (and sometimes real people) doing sexy things to each other.
You will probably laugh more than getting excited, but you will leave most of those tales with a smile in your face. And let's face it, everyone, including you, have already fantasized about two characters you love or people you know doing the sexy time.
Stop denying it, you know to be true.