Well, you can. The question is should you? More importantly, what is God asking of you in this situation? Mostly consider refraining from sex until after the wedding. Even if you are now in a “committed relationship” you are still immersed in a culture whose first commandment is, “If it feels good, do it.” Maybe the idea of “Wait! It will feel and be even better!” is worth considering.
There is so much wrong with the hook-up culture in which so many young people try and form their minds and hearts and prepare for marriage. But the worst thing about hook-ups (solely physical sex without any expectation of any further connection, let alone relationship) is that such encounters make sex so much less than it can be, i.e. a deeply transformative act of love and communication that makes us who we are to be for all eternity. To settle for hook-up sex is like settling for wonder bread and flat soda for dinner when a seven course meal with excellent wines at a five star restaurant is waiting for you.
Secondly, with regards to the wedding, we perhaps might want to think of how we can pair down the cost for the wedding. Do we really need to have an actual “boatload of shrimp” as I saw at a recent wedding reception? What do we truly value?
Some couples actually live as “brother and sister” during engagement because of financial constraints. It’s not easy but some do so. Some who are “living together” actually decide before God to refrain from sexual relations while preparing to enter into marriage.
Kurt Warner the great pro football quarterback fell in love with a lovely woman. She thought he would be turned off by her two children, one of whom is severely disabled. Kurt actually grew a great deal through his relationship with her and her kids. He also rediscovered his Christian faith. In reading the bible, he became convinced that he and his future wife should refrain from sex until they were married in the church. They lived that way for a year. Grandma kisses were their limit. It can be done (cf. All Things Possible by Kurt Warner, 2001, Pp. 115-116).