We should be telling them what the Bible says about marriage. From the earliest age, we should be telling our children that marriage is a commitment to God, a recognition of His social hierarchy and His purpose for marriage which is to give glory to God; to serve Him with all our heart, mind and soul... not to serve our own ideas, wants and or desires.
It is not enough to talk to our children, we must visually express this in our own marriage. We must show them that it is by God's authority that we live to love and serve the Lord in marriage. You see, marriage is a not means of finding happiness through another person because one thinks that he or she is the 'right person' as in the one of your dreams and the one who can make you happy. There should only be one dream and that is to love and serve the Lord our God with all your heart, mind and soul.
Americans go at marriage the wrong way. They think that there is 'the person' out there that God made just for them. Of course, we can look at it that way and feel good about the person we married thinking that God created just that person for us but God did not make someone special just for you, he made people for Him. He is not god - the a creator of individual personalities. He is God the Creator of all things seen and unseen. His plan for His creation is much bigger than one can imagine. He is spiritual and He put that divine spirit in each one of us in order to grow in spirit and that is as special as it gets which is very special.
We need to seek His guidance in the decision to marry and in marriage so as to move forward in faith not in a 'happy marriage' as if that is the only purpose for marriage ~ to be happy. For it is not. Tell your children to that, then you/they will know that the person they are thinking to marry is the right one in terms of a godly choice. What about love? A person can and should love all people in Christ Jesus. This is possible as it is God's true wish for us for it glorifies Him. Marriage is not about finding your 'true love' though you can love someone dearly and truly... in knowing that God is our true love! Tell your children to put God first and ask Him for guidance when considering to marry for marriage is a sign of moving forward in faith and in God's purpose.
As for which gender you marry. Firstly, know that "...the wicked will not inherit the kingdom of God. Do not be deceived. Neither the sexually immoral nor idolators nor adulterers nor male prostitutes nor homosexual offenders nor thieves not the greedy nor drunkards nor slanderers nor swindlers will inherit the kingdom of God" 1 COR 6:9-11. "Keep the marriage bed pure for God will judge." Hebrews 13:4
The Bible is clear about marriage between a man and a woman. God made people in His image, and He made them male and female different and yet in marriage "The two will become one flesh" 1 COR 6:16.
All in all "... it is good for a man not to marry. But since there is so much immorality, each man should have his own wife and each woman her own husband." 1 COR 7:1-3