I'm 18 and my boyfriend wants me to have sex with him for the first time and he is 19 and we are getting married soon should I wait until we get married or before we get married?

If you have already decided to get married, then why not wait until then.
There's no need to rush into things..

What reasons is he giving for not waiting?

Ask yourself what it is you want to do?

Did you always think it would happen on your wedding night?

Talk about it together, if you can't talk about this its going to be the least of your problems.

If you have waited this long, you might as well go the traditional way and save it for the wedding night. No one gets hurt and nothing gets spoiled.

Good Luck,
Javi