The person you sleep with should tick these three boxes

Box 1: They are your spouse.
Box 2: You are married to them.
Box 3: Refer back to box 1.

This, obviously, isn't the reality for everyone.

People have sex with who they want, for a variety of reasons.
But think the following is true no matter what you believe.

For, no matter your worldview, I think it makes the most sense to have anyone you sleep with tick the following boxes...

Box 1: With sex being a magnifier physically, emotionally and relationally it pays to know the person beforehand. Strangers, generally, don't make good bedfellows.

Box 2: Know why you BOTH want to have sex. Is it just casual? Will it just be physical? Are you hoping it will lead to more? Is it about power or control? Both deserve to know the foundation of the request.

Box 3: Be with someone who will give you the best response for BOTH of you physically, emotionally and relationally - depending on the background you have together, the baggage coming in and the messiness of a sexual relationship between the two of you. In some cases, the wisest and most caring thing will be for one of you to say no. Or at least not now.

Ideally, these three boxes are covered within the context of commitment, faithfulness, trust and love... Marriage.

You know the person better than anyone, sex is a sign of your commitment, faithfulness, love and dedication and it won't happen until the previous things are in place.