Sagittarius Rising and to a lesser degree Sagittarius Sun or Moon people are prone to more difficulties in relationships.
You have heard many people say how hard it is to keep a smooth relationship with a Sagittarius(especially Sag Ascendant ppl). Modern astrologers say this is due to the adventurous nature of Saggies, their need for freedom, etc. But this is not the reason.
Sagittarius is prone to difficulties in love because Venus-the planet of equal relationships rules the problematic 6th house/sign from Sagittarius. 6th house rules inequality, arguments, conflicts, enmity. Merging the meaning of this with Venus, we see why the relationships of Sagittarius go through a fair share of such issues.
6th house rules the employee, the sick, the underdog, the victim, the conflicted, the addicted, those in debt or those in trouble. It has been observed that people with Sagittarius ascendant to a lesser degree Sun or Moon, can be attracted to servants, someone who works for them(is below them on the career ladder), who is from a lower social class, has financial problems(debt), is sick, has some addiction, is troubled in some way or is more conflict-prone or critical(all things indicated by the 6th house).
On the positive side they can attract a partner(Venus) who manifests the higher side of the 6th house and is very hard working, is involved in some sort of social work or medical field helping others with problems, is in the banking or debt regulating industries, solves problems/conflicts-litigator, police force, military, etc
But even in this case the relationships of a Sagittarius have more than a fair share of stress, animosity or out of balance, unequal bonding.
6th house also rules pets, so Sagittarius Asc/Sun/Moon natives are know for their love(Venus) of pets(6th house) and kind, helpful attitudes(Venus) to the weaker, the servants, the employees and people from lower social classes.
Some Male Sagittarius, have been observed to have attraction to problem(6th) women(venus) and even prostitutes.
But this is also the wonderful charming side of Sagittarius, they can show a lot of kindness and support(Venus) towards those "below" them and the downtrodden, that is why they are quite philanthropical.
6th house rules one's duties, debts and everything which requires hard work. When the planet of love-Venus rules this house, as in the case of Sagittarius, relationships can be(or feel like) a burden or too much responsibility/hard work.
It is not that Sagittarius does not like commitment or is unstable in love, it is just that this sign has more karmic debt when it comes relationships so they attract more difficult such. Relationships and service/duties/routines come in a package for freedom loving, Sagittarius....
Idealistic Sagittarius clashes with the practical, repetitive mundane demands of the 6th house which is ruled by the planet of love in their horoscope.
6th house also rules health problems, often Sagittarius might have nervous/mental and even physical health problems because of relationship stress.
the 6th house improves with age-the older one gets the more they master it, this is the good new for the relationships of Sagittarius. They can expect more partnership bliss later.
But Sagittarius can also attract relationships which have the nature of the 11th sign from it, which is the other sign ruled by Venus(love, relationships). In this case the spouse/partner will have a wide circle of friends, be socially active, well connected, gainful etc, but the 6th house will continue to exert an influence bringing its more complex sides.
On another level 7th sign from Sagittarius shows the partner too. For Sagittarius the 7th sign is Gemini. Gemini is ruled by Mercury and Sagittarius by Jupiter. In ancient Astrology Mercury is an enemy to Jupiter.
Jupiter(Sagittarius natives) tend to look down on Mercury(the partner(s)) as a minor player, adolescent in behavior, restricted in view due to immaturity and excessive mentalization, not capable of wisdom-knowledge, impressionable, short-attention-span, calculating but not comprehending, frivolously chatty, and often quite annoying.
This can be especially true if a broad minded Sagittarius native has a more superficial spouse/partner whose philosophical scope of awareness is rather limited and busy with superficial details rather than the big picture.
The second marriage/marriage like relationship of Sagittarius will be usually better than the first, and there is often more than one marriage(especially for Sagittarius Ascendant) because Gemini(relationships) is a dual sign.