For relationship planets, the placement of Venus and Moon is very important in a man's chart in terms of what values and type of relationships he is looking for in a female. In women's charts, the Sun and Mars are traditionally considered the most relevant planets in terms of what characteristics the female is seeking in a man.
The moon is central for both sexes, in determining what kinds of emotional security the person is most likely drawn to.
I was once in a short relationship with a man who had a lovely Venus/Jupiter conjunction in Gemini. Not only were these planets conjunct my Ascendant, but they were trine some of my personal planets in Aquarius.
I thought we would have a very rosy relationship. But I've read that men who have a Venus in Gemini are often not happy with just one female in their life. More often they need two. They are wonderfully poetic when it comes to verbal romancing, but they can be flighty and always on the lookout for the next interesting female to verbally seduce. This was certainly true of my ex-boyfriend, but I'm sure it doesn't necessarily apply to many men with Venus in Gemini. Isthmus Nekoi has written an excellent summary of the kind of values that each sign associates with love and relationships. See Nekoi's article here.
See also about how to determine if there is compatibility between two people based on their astrology charts.
