Where Marriage is Shows up in astrology?

First thing, be it Rahu Ketu Sun Saturn Mercury Venus Mars Moon Jupiter, no planet is in the 7th house has the possibility to deny marriage. There are specific results ascribed to the nature of married life for all the planets in the 7th house individually and all of those can vary from anything between good and bad. Each planet in the 7th can give good or bad marriages subject to the specific combination. And if you collect even 500 horoscopes, you will get at least one married human for each of the planets influencing his/her 7th house.

It is also a lot of hogwash that strength or weakness of planets/houses will deny marriage. Denial is not a result of weakness, but of complete absence of results. A weak house can give varying results, such as distance with the spouse, ill-health to the spouse, divorce, short-life, delayed marriage, etc. But for marriage to be denied completely, strength is irrelevant.

First of all, to understand the concept, you would need to break it down into possible social situations. And then it would be easier to understand the astrological combinations.

Denial of Marriage
Marriage but denial of any marital bliss (namesake/on-paper marriage)
Relationship without Marriage
Social obstruction to marriage (homosexuality, in acceptance due to cast, creed, race, religion, etc.)
Ascetism
Each of the five is a different case and has different combinations attached to it. However, each of those have very clear, strong and accurate principles associated with them. I’ll cover these one by one.

Denial of Marriage:

Denial of marriage here means neither marriage, nor any physical relationships; or else, no marriage but near-about no physical relationships. Here, the 7th house, and karaka of the 7th house needs to be afflicted. However, what people miss often is the fact that Vedic astrology does not promote using the lagna alone as the center of the horoscope. And hence, the rules do not seem to work for most people, because they’re always looking at 1/10th the picture.

Marriage is governed by the following factors:

7th house
7th lord
Venus
7th house from Moon (not the lord)
7th house from Venus (not the lord)
Navamsha placement of 7th lord and Venus.
And the rules says that if two of more natural malefics afflict a house at close proximity, if the lord is retrograde, if natural benefics do not aspect/protect it (or if the protecting benefics are retrograde); the results of the house will be denied to the person.

However, the rule changes when you miss considering all the houses associated with marriage. If the 7th from lagna is badly afflicted, but if the 7th from Moon and Venus are strong, the result will be that either there are emotional distances with the spouse or the spouse has health problems.

Ideally you’ll see that when marriage is denied, the most cruel malefic aspects converge in one or two houses which are unprotected by benefics. And those one or two houses alone cover most of the above six points. For example if Mars, Saturn, Ketu and Sun all aspect Aries and afflict the sign then we can say for sure that the results of the house would be denied. Now, lets say the aspecting Mars is in Virgo, Saturn in Aquarius, Ketu in Leo and Sun in libra. So effectively, all malefic aspects converge only on Aries. Now in such a horoscope if Moon and Venus are combust with Sun, all the above criteria is met. Moon is combust and its 7th house (Aries) gets afflicted. Venus itself is combust and 7th from it is afflicted while Rahu aspects Venus. So now, the chart qualifies for denial of marriage and the fate would be sealed if Jupiter is retrograde. If not, its placement may give a very late marriage in the late 40s.

Marriage but denial of any marital bliss (namesake/on-paper marriage)

Afflictions leading to denial of results of the 12th house and influence of impotent planets on the 7th house. Now, for reference in such cases of sexual capability, Mars, Sun and Jupiter are masculine, Venus and Moon are feminine while Saturn and Mercury are impotent. Rahu and Ketu can be considered similar to Saturn and Mars respectively. If 12th house is afflicted and 7th lacks a gender aspect, physical partnership in marriage would be denied. If Venus is debilitated, it seals the fate. If 8th is afflicted, it is due to lack of sexual virility. If other houses are afflicted, reasons can change.