Are you asking if his interests are a phase or are you asking about him taking things? I suggest that his interests are probably not a phase tho they will change over time. He likes "girly" stuff at the moment. He may learn to enjoy sports later.
If you're concerned that his interests are an indication of his sexual orientation I say that they may be but also they're just as likely to not be. Six is too early to have a sexual orientation. Six is still an age without sexual instincts. Six is when kids explore everything but not sex.
When you say he takes girls stuff from class are you describing behavior that is actually stealing. Does he take the stuff without permission? I would be more concerned about stealing than his interests if he's taking stuff without permission. He needs a loving immediate consequence that includes returning the item to it's owner.
Is it possible he's taking girl's stuff because he doesn't have the things he's interested in. Sometimes parents are so concerned about a boy being gay that they only give him boy's stuff. If he doesn't have girly stuff for himself, I urge you to give him toys that are of interest to him.
If he's going to be gay there is nothing you can do to prevent that. We are born with our sexual orientation. The healthiest thing you can do is to allow him to have his interests and wait and see. Be assured that it's most likely that he's a normal boy with an artistic bent. Manly men also enjoy music and bright lights.
I've known several boys who enjoyed sports and bright lights too. Bright lights are attractive to most of us. As far as jewelry is concerned, I've also known young boys to enjoy dress up including wearing a skirt and jewelry. They're exploring the world which does include girls and "girly" things.
Enjoy your son as he is. Continue to provide him with a variety of things including the "girly" toys in which he's interested. If he has his own he's less likely to borrow (steal) others'. His interests will expand with time.
I'm always glad to see boys interested in so called girls toys and girls in so called boys toys. This produces a well rounded and interesting adult who fits into a variety of milieus (places). You mentioned music. Think of some of the singers who are boys and men. They aren't gay.
Your son is OK. He needs you to be interested in and share his interests. Look at how pretty the things he likes are and stop equating them with sexual orientation, if that is what you're doing. Teach him boundaries surrounding ownership and insist that he ask to borrow stuff from other kids. Provide him with his own stuff so that he can share with them as well.