It's not understandable at all, and attitudes like yours is why they behave with such idiocy. There is a global pandemic, and so many adjustments and hard work has happened to create 'biosecure conditions' to allow these players to do what they love, which is play competitive football. And yet we then get these two idiots willing to risk the health and safety of the entire group, just because they felt horny.
Foden and Greenwood especially are 'jewel in the crown' academy graduates from their clubs, whom have been educated in every facet of being a modern day footballer in English Premier League. Lack of judgement like this simply shouldn't happen. They have self created a shitstorm around themselves, which I doubt they are emotional equipped to handle.
Im not sure what disciplinary measures the clubs should take, but I hope it's swift and harsh and comes from England and their clubs. Let it be a very painful lesson