Think about what you want to do in your life. If you get engaged, discuss this with your future spouse. Talk about it with friends. It is important not to look too much at others, and look instead at Jesus and the Saints. Actually, the Church does not consider this a sin, but people should listen to their consciences.
Many people marry for the wrong reasons or no reasons at all.
When they are uncomfortable enough, they separate. Confess that, and change your life, with God's help. Then you won't have to worry about STDs. They both committed adultery. They are not getting married in the Church but at the beach.