Franklin is indeed the straight one. But the case of an all-straight group every episode isn't true. Episode 3 (three-man groups) didn't have an all-straight group.
In Episode 2, Jim, Dan, Franklin, and Sean (all straight) are paired, sending three straight men to Episode 3. Group 2 was Wes, Michael, Rob, and Paul (2 straight, 2 gay) sending at least one straight man to Episode 3. (Group 3 was all gay.)
Since only four straight men (vs. five gay men) made it to Episode 3, the show had to ensure that at least two of them continued. It could have done so with an all-straight three-man group, but it didn't. The show was a little craftier than that and chose instead to have two groups contain two straight men each, ensuring that at least one straight man from each of those groups advanced. Plus, it added to the possibility that all four of them could have advanced. If this had been the case, of course, the two gay men would have been paired in Episode 4 so that James had at least some shot at happiness.
Amazingly, with the elimination groups, James has been picking off the straight men with great accuracy, doing so every time he's gotten the chance. Every (and I mean every) gay man that's been eliminated from an elimination group came from an all-gay elimination group and James had no choice. Having only two straight men in Episode 4 was the worst-case scenario for the show, but they planned and prepared for it by pairing them together.