Once you get a Girl to a Pink Heart and have a Kitchen, a Blue Feather will arrive in Rick's Tool Shop. Buy the Blue Feather and sprinkle it over the Girl that has a Pink Heart. You'll get married the next Sunday.

After you're married, your Farm Life changes. Some mornings, your Wife will do some chores. Each Girl's chores are listed on the Girls page. After you buy a Baby Bed, you can get your Wife pregnant. To get her pregnant, make sure to go to Bed between 8:00 and 9:00 P.M. and give Gifts to her and only her. One day, the Midwife will appear at your House so your Wife can give birth to your Baby. Your Baby has Three Stages. The Stages are:
Infant Stage
Crawling Stage
Talking Stage
Each Stage lasts a few Seasons. You need to keep your Baby happy, so give it Gifts, show it your Dog, and carry it.

While married, you can still give Gifts to the other Girls, but for every Gift you give one of the Girls you didn't marry, give two Gifts to your Wife to keep her happy. If she gets displeased enough, she'll you and move back to her parents' house until you make her happy again. While she and your Baby are at her parents' house, the other Girls will disappear. Make your Wife happy enough, and she'll move back in with you. To keep all of this from happening, just keep your Wife happy. To keep your Wife happy, make sure you dance with her at the Festivals that allow dancing, lavish her with Gifts, and only occasionally talk to and give Gifts to the other Girls.