That honestly depends on what you think, but I would definitely say there is no problem with it.
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it depends on which type
No, I know straight people who are girls who watch lesbians make out and have sex. If you are a guy, its completely normal. For a girl, its normal, too. It doesnt matter what sex you are. You are allowed to do anything.
Touhou
I'm Pretty sure the anime you are looking for is called School Rumble. I might be wrong, but it does have a bad boy who falls in love.
Anime doesnt neccesarily use the ratings scale but for your question anime that 'would' be rated 18 above is any ecchi or hentai anime.