Why do some people want to do it when they're not married?

Making love is one thing, having sex is another. Most people expect some kind of commitment before the relationship goes to bed. Some would rather assume the commitment is there and go ahead and have sex. It goes both ways. It is your choice how far you are willing to go. Communicate that boundry to your mate and hold each other to it.

for rthe fun of it i guess and alot of people are in love wen the have sex but just dont want to get married

Proably for the thril lof it!

Assuming by “it” you mean sex.

Most people find sex pleasurable. Since people are marrying later and are going through puberty earlier they are often sexually mature a decade or longer before they marry. Asking sexually mature individuals to hold off on sex for this long is impractical. Sexual compatability is an important part of marriage. It isn’t the most important part but it is important enough to check into before promsing to forsake all others. Many marriages are stressed from sexual incompatability. Perhaps one partner wants sex daily while the other is happy with sex a few times a year. One partner may be adventurous sexually while the other just wants straight ahead missionary sex. People should know what they are getting into before tying the knot. Sex before marriage also avoids the situation where one or both partners are sexually inexperienced and awkward.