Lupin is a globe-trotting thief which makes it rather difficult to build meaningful relationships with women. You can perhaps relate his situation to that of another philandering gallivanter in James Bond. Bond is another who is skilled when it comes to women (no doubt through years of experience) and flits from one to the next at the drop of a hat. When Bond did try to create a stable relationship with a woman, it ended in disaster. The same sentiment could well apply to Lupin who is happy to have dalliance after dalliance, but never commits to a relationship. The only notable exception is his, well, sadomasochistic affection for the well-apportioned Mine Fujiko who is part of the same world as he is.
The goofy character is just something that makes the anime entertaining. FWIW, neither is he all that goofy in the manga nor is he particularly chivalrous. His WP page elaborates thus:
In Monkey Punch's original manga, Lupin is cocky, quite crude, and for the most part remorseless. He is very much the ladies' man, often using them for his own gains, but is not beyond forcing himself upon women who resist him. This is in stark contrast to his better-known anime self (excluding the fourth series), who despite being a skilled thief, comes off as a goofball that will go to great lengths to right injustice, who also shows a chivalrous streak that compels him to help those less fortunate than he.
The goofy, lecherous persona is also a very 70s/80s thing and can also be seen in some of Lupin Sansei's contemporaries such as City Hunter.
