You don't need some specific topic to become rivals over. The fact that they get competitive over things is what drives their "small rivalry aspect". A rival is literally just someone you compete with.
I see Tomoko talking about Nemo's genitals to her face as more of a point towards Tomoko's social awkwardness rather than a point of sexual tension between the two, if that's what you're referring to. Personally, I don't think it really compares to Tomoko and Asuka constantly talking about (and in rare cases acting upon) touching each other's various body parts or Tomoko and Yuri acting like a newly formed awkward couple all of the time. But to be fair, that's mostly up to interpretation, so if you see it another way, fair enough.