Actually everyone is capable to cheat their partner, no matter the sun, venus, moon, mars, juno, vesta, or 7th house sign. Marriage is just a symbolic, just a contract. Its a decision made by two people about living a life together where both have to make a little effort to please the other one. If one of the partner's fail, then things can lead to make one of them cheat.
Imagine you are a guy and you arrive home after 10 years of marriage, and you see your wife getting fat, she quitted wearing make up and washing her teeth, she doesn't has sexual appetite anymore, and she is in a terrible mood all day long. Guess what would you do when a beautiful young woman starts flirting with you? The same if your guy gets fat, and spend all days drinking beer at the sofa watching TV without spending time of quality with you. Adding that he is a control freak that doesn't let you go out sometimes. Its just a huge turn off.
Anyone who isn't happy with what they are getting will cheat. So any sign who is not happy with the partner they have and they see another one who brings what they desire... They'll cheat and even left their actual partner for another one.
Of course there are astrological signs wich are more impulsive to do so (Venus/Moon/Mars in Aries or Sag), or who are able to lie more easily (Venus, Moon, Mars in Gemini or Pisces), or who have more sexual appetite (Venus Moon, Mars in Scorpio or Aries) or signs who give a lot of importance to physical beauty like Taurus or Libra and maybe Leo, Or signs who are very picky with hygiene and weight like Virgo, Capricorn and maybe Aquarius.
There are other signs who think it twice before cheating cause they are more passive Like Cancer, Taurus.
Think this.... Let's say the most general thing:
The most loyal= Cancer, Scorpio, and Taurus.
The most unfaithful= Gemini, Sagittarius, Aries
If someone who owns Cancer, Scorpio or Taurus in its Sun, Moon, Mars or Venus, and their partner doesn't fulfill their expectations of loyalty, commitment, passion, and home-loving (attributs that this signs expect) Then they'll cheat.
If someone who owns Gemini, Sag, or Aries in Sun, Moon, Venus or Mars, and their partner is suffocating, does't allow them space, its very passive, then they'll go for the first person who meets those expectations.
Hope this helped you.