It's said the best way to train your Pokemon is putting two of them to train together, so they create synergy and power up faster.
Oh, why they're having sex? Hm... It's related to what I just said. Sy-ner-gy. Don't think too much about it. There's science behind it and several papers made by respected trainers of humanoid Pokemons aprove.
Also, I should draw more scenes in locker rooms... There's some untapped potential right here for casual encounters in gyms or showers... Well, definitively is going in my list. 😋