There is a medical explanation for this: Semen contains hormones which are known to function as anti-depressants. On the other hand, women in general do not like condoms. They report that condoms make them feel "icky" or "yucky".
It has also been my personal observation that when a woman reduces or withdraws from sexual activity, she often becomes depressed, becomes fat or overweight and no longer tries to look her best or to take care of herself. Two of my former partners are dramatic examples of this, the most recent being Passion Julinsey, who started putting on weight as she cut down on sex with me. I attributed this to lack of exercise (sex is good exercise), but the semen/depression studies seem to provide a better explanation.
Here are two recent reports on this subject:
New Scientist Reports: Semen Acts as an Anti-Depressant
EurekAlert: Semen makes you happy!