I'm pretty sure that once you've been "caught" visiting the lady without permission by the lady's father, the father will say something like, "I've heard that you've already been visiting her and I won't let you have her after seeing her behind my back. We will discuss this no more" and you get a -1 relationship.
After you've been "caught," you cannot get that father to approve of your marriage no matter what. You're just **** out of luck. Go for another lord's daughter. Try to find the most prestigious lord in your kingdom and go for his daughter...good stuff there.
If you can't obtain a father's permission to see a lady right away but you've already done the "I'm your most ardent admirer," thing...then you can still visit her whenever the lady sends a message and you get the quest. Only attempt to visit her if you have the quest; if you don't have the quest, you'll get some kind of message like, "The guards scowl at you and you're unable to visit the lady." Once that happens, I'm pretty sure that the father discovers you paying attentions to her. In addition, only secretly visit her a couple of times--I think you run the risk of being caught the more times you do it. So build up your relationship with the lord and your renown as quickly as possible.