No, it’s not that you must have children. It is that you must be prepared to love and care for any children that are given to you, preferably with joy. Children are not the sole purpose of marriage, but they are inextricably linked with it.
Look at it this way. Marriage is a sacramental connection of overflowing, reciprocal love. Children are one of the results of that overflow. If you go into marriage intending to never love to that degree, you go into marriage intending to never be married.
You can time the marital act so that children are extremely unlikely. You can even do so for indefinite periods of time. But if you reject the idea of motherhood and fatherhood entirely, you reject marriage.
As for who you have to date or marry, I’ve known plenty of Catholics who dated and married non-Catholics. There are just special forms to fill out so the Church can make sure the marriage is sacramental and you will have to promise that you (you specifically, not them) will do your best to raise any children Catholic.