Flee also youthful lusts; but pursue righteousness, faith, love, peace with those who call on the Lord out of a pure heart (2 Tim 2:22)
This is just my opinion, but if God’s number one in my life, I’m going to do everything I can to please Him, in my relationships and everything else. There’s obviously nothing wrong with getting married. "It’s better to marry than to burn with passion." (1 Corinthians 7:9) But Scripture over and over warns against fornication, and commands us as Christians to dwell on what is pure and upright. (I guess I've never struggled with this, but I'm sure even just simple kissing could be a stumbling block for a lot of Christians. It doesn't necessarily invoke 'pure' thoughts...)
Also… We all know that we're supposed to’ love one another as ourselves’. If you really, truly love someone, wouldn't that motivate you to strengthen them in their walk with Christ? Will your kissing them before they’ve married either you or anyone else make them a better person in the future?