Very weird choice of skill from the FMC. Previously, she was thinking that they had great compatibility, which means she was at least content with their bedroom action, if not straight happy.
What happened is that she ruined the dynamic of their relationship by picking this skill, as there was an equilibrium of their vitalities and now no matter how hard the MC tries, he cannot satisfy her. Which means we get the unhappy/unconfident MC and his unsatisfied, sex hungry girlfriend. So now he's forced to seek solutions to keep her satisfied instead of thinking about their future wellbeing.
Basically, her actions only benefit her now, because in case something goes south, no matter the source of semen she receives (or the number of sources), it still benefits her, as she became a kind of succubus at this point.
The only situation I think her choosing this skill would be justified, is if the MC eventually gets a huge enough amount of vigor while gaining XP for closing his quotas and she becomes no longer able to satisfy him properly. And it's not the case here.
I'm greatly disappointed now. Will see how things unwrap, maybe I'm wrong but I was much happier with how they were before.