Well, people don't need to be lovers with person they fuck behind their partner's back, in order for it to be cheating. "He/she is just my friend with benefits" isn't going to cut it as explanation. It's sexual activity that generally counts, whether there's any deeper feelings involved or not.She doms her, but they're friends, not lovers.
Also, i think Louise would strongly disagree with the idea she and Lena are "just friends" at this point. Much like she was quick to consider Jeremy her boyfriend.