I am a strong Sagittarius (stellium of planets in Sag), so I can identify with the "sag" energy very well.
To be really honest here (another sag trait), I think the problem may be with the question you're asking, and not with the other person you're asking about. For example, if you take a time out and think about it, what is it that you're really after here? Are you trying to find a way to keep your love with you, and only you, forever? That's a perfectly valid desire, as we all want that, but the reality is that we can't control other people or make them love us the way we want to be loved. Not to mention, how stressful is it to spend so much of your life energy trying to force a particular outcome (like "keep" someone from cheating), when your limited every can be used for more positive and loving channels...like loving yourself! If this other person with Venus in Sag is not meant to be with you, then the universe will reveal that to you in someway.
Regarding your actual question, I second the motion from other posts that you can't tell what someone is going to be like from just their venus sign. For example, I know a Venus in Cancer who is horrible at staying faithful, but that is due to a lot of other aspects in his chart AND the nature of his progressed chart.
Sagittarius is about exploring, discovery, fun, faith in goodness and life, adventure, humor, light-heartedness....that is the kind of energy that a Venus is Sag find attractive. But that doesn't mean he/she will cheat. For example, religion fall under Sag too (faith), along with higher education (philosophy). I know a lot of Sag folks who express this side of themselves through these other interests.
So my opinion is that cheating is not exclusive to "Sagitarrius", in the same way that "Pisces" does not always mean drug addict. The two concepts you are putting together are separate and much more complicated than how your question is presented.
I don't know what your personal situation is, so I won't judge. But my advice is to spend more time understanding your own chart and personal needs for a satisfying and fulfilling life.....and once you begin to fill those needs of your individual soul, you will attract the right kind of love/partner.