That letter was interesting for me to ready because I am 23 and still live at home (six more months, then I'm OUT!). I agree with the people who said Abby took the easy way out. Things could be worked out differently than she suggested.
My story: My parents are Catholic and do not believe in premarital sex or living with someone before you get married. They are, however, realistic about things and know that they have to let their adult children do what they feel is right. I have a boyfriend whose house I sleep at every couple of weeks or so and I have gone camping with him and gone on "overnight" trips with him. I basically have a "don't ask, don't tell" policy going. I don't say, "Goodnight! Sleeping at my boyfriend's house tonight!". I leave it at, "I'll be staying out tonight so I'll see you in the morning." I'm sure they know where I am but we don't discuss it because I know how they feel and they know I'm old enough to decide for myself.