The Sun Moon Midpoint in astrology, and more specifically in the natal chart, is a highly sensitive and personal point. It is a mathematical point--the point midway (or exactly halfway) between the Sun and the Moon.
Inherent in the symbolism of the Sun/Moon midpoint is the desire for companionship. The Sun/Moon midpoint represents a point of "inner marriage" and symbolizes close personal relationship. This point is very important in any kind of relationship analysis, in addition to natal chart work.
Note that with midpoints, it is customary to consider other planets and points that form the following aspects to the midpoint: conjunction, opposition, square, semi-square, and sesquiquadrature. When a planet makes any of these aspects to a midpoint, the notation looks like the following (where, for example, Venus is the planet forming one of these aspects with the Sun/Moon midpoint):
Venus=Sun/Moon
or: 
=
=In synastry, the Sun/Moon midpoint is very often activated by the other person's personal planet in significant relationships. If another person's personal planet, and especially the Sun or Moon, forms an aspect such as the ones referred to above, that person represents some kind of promise of integration to the Sun/Moon midpoint owner. Whether or not integration actually happens, this synastry connection offers a promise which can be difficult to resist. Therefore, it is a strong indicator of attraction. Many astrologers assert that significant relationships that lead to marriage or a similar commitment contain at least one of the following interchanges in synastry: one person's Sun aspecting the other's Moon, and/or one person's personal planet aspecting the other's Sun/Moon midpoint.
Note About Midpoints:
In order to understand the symbolism of the Sun/Moon midpoint, one must understand the symbolism of midpoints in general. Ebertin, respected as an authority on midpoints, defined the midpoint as a sensitive point in a chart where the influence of the two planets involved combine together.
Cafe Astrology
The midpoint in your chart between your Sun (masculine principle)and Moon(feminine principle), is a very sensitive point in regards to marriage and romance. Any potential romantic partner with a planet on your Sun/Moon midpoint is going to have an effect on your romantic life or your marriage. Your potential mate with his or her Sun or Moon on that degree will definitely have an impact on you, you are not imagining it.
This is also sensitive to any opposition and square to this midpoint degree. So all in all if you find a romantic partner does have their Sun or Moon conjunct, square or opposing your Sun/Moon midpoint it is not all in your head, you are not imagining the attraction it is there.
The Sun/Moon midpoints are also active as marriage timers, when they are contacted or activated in transit by the outer planets Pluto, Neptune, Uranus, Saturn or Jupiter. The transits are generally activated by conjunction, square, or opposition. This degree is also sensitive to progressions at the time of marriage.
http://www.astrologicalpredictions.ca/news/sunmoon.htm
