Everyone feels comfortable with different things when it comes to sex. Communication is a really important part of any healthy relationship, sexual or not. Talking about what you do and don’t feel comfortable with (and like or don’t like) is a chance for you to be honest with each other and become closer.
Generally, you should try to talk with your boyfriend/girlfriend about sex and what you would like when you’re both comfortable and thinking clearly.
Be respectful of their needs, too. They have the right to decide whether they feel comfortable doing what you want, and you should respect their wishes just as they should respect yours.
If they want to do something that you don’t want to do, suggest something else you would rather do instead. You never have to have sex that you don’t want to have.
Doing sexual act agaist your partner’s agreement is the crime.