I did this one when I was getting "The Spouse" Achievement. I did the following to get here to marry me.
Click Spoiler to see how to get someone to marry you.
*** Spoiler - click to reveal ***
When you accept a Side Quest called "Till Death Do Us Part" from a ghost. He will ask you to go to the market and seduce "Alex" you can either give her the note like the ghost asks or you can Marry her yourself.
To get her to marry you its simple, Keep using Thumbs up, Flirt Expressions and Gift the books the ghost gave you to her.
Once the bar it full enough and she has a RED HEART next to hear head then you can ask her to follow you and take her to the jewelers in the market and by one of the 2 wedding rings.
If the RED HEARTS disappeared by time you've done this Flirt with her again to get it to come back and the Gift her the Wedding Ring.
Once she says yes it will tell you to take her to your house and click on the Sign out side to complete the marriage.
Once you married and at your home if you get her to follow you using the Expression Menu and go up stairs to the bedroom, Click on the bed and select "Unprotected Sex" the screen will go black and she will says something like "You've Done it I'm pregnant." and Achievement will unlock.