I mean. What's Henri's other choice? Assume that his ROOMMATE likes him and totally destroy their friendship and living arrangement?
Also, being intimate with someone, even sexual with someone, isn't either romantic or using.
Platonic sex exists. Experimentation exists. Henri is a person Levi trusts, and Henri would know that. Is it using someone to be that for them?
And these points are ignoring the fact that Levi started the whole thing. From the very beginning. Levi was even the one to bring up the gay stuff.
As for looking like Piet? Levi and Piet have very little alike outside of their appearance, and Henri knows both of them pretty well. While I'm sure there IS some physical attraction that might exist within Henri because of his attraction to Piet, I don't think that would do anymore than allow Henri to explore feelings he might not otherwise explore.