If she's not attached, what's wrong with it. When I was single I had sex with three guys in one week. It was wonderful. I'm married and settled now, but when you're young why not enjoy your sexuality?
YOU DECIDE. If you're asking, you're probably not ok with it. Some people can do recreational sex, some can't, some are sorta off and on, ect. But the trolls that judge you are to be ignored.