Answer

This is not a question of permitted or forbidden. The question is whether the condition is valid or not. The Talmud indicates that there is no effective condition in marriage, meaning that if a condition is imposed in the betrothal, it is nullified at the time of the marriage itself (because a person does not make his sexual relations into promiscuous relations). But this is really only a question of final intent, meaning that if it is clear that the person does not want the condition to be nullified, there is no reason to assume that it becomes void with the marriage.
However, the halakhic decisors are uncomfortable with someone who imposes a condition on his betrothal, especially if he continues it into the marriage, for various reasons. But those are reasons of halakhic policy, not truly halakhic reasons. See a relatively brief overview here:
http://www.kipa.co.il/%D7%99%D7%94%D7%93%D7%95%D7%AA/%D7%9E%D7%95%D7%AA%D7%A8-%D7%9C%D7%91%D7%A6%D7%A2-%D7%A7%D7%99%D7%93%D7%95%D7%A9%D7%99%D7%9F-%D7%A2%D7%9C-%D7%AA%D7%A0%D7%90%D7%99/