1/month sex with the SO before the pandemic doesn’t mean she is cheating. Not regularly anyway. As the OP stated there are 3 kids to attend to as well, and being a stay at home mom, she is just exhausted imho. Improved libido during the pandemic is probably tied to OP taking on some of that work over and her having more energy. Plus, as long as the relationship was ok in other respects, being locked in probably improved feeling of intimacy for her, hence another boost to libido.
And if she has a side piece, As long as the relationship is fine outside the bedroom, it’s probably best to let her work her shit out. From cost benefit perspective (in OP specific situation, ie kids, house, good job, etc), it’s kinda dumb to get divorced based on bedroom issues alone. Especially since OP can get that need taken care of when he really needs to.