According to the site at the bottom of this post, Nasal sprays containing oxytocin or a placebo when administered in an experiment promotes monogamy by
preventing men with partners from "signaling romantic
interest" to other women.

METHODOLOGY : The research team singled out
the most attractive female among them to
approach their male subjects.

Each of the 57 men
had been administered either oxytocin or a
placebo via nasal spray prior to the encounter.
The attractive researcher would stand about 24
inches away from the subject, and then move
toward and away from them.

The men were
asked to determine when the attractive
researcher was at an "ideal distance" and when
she got too close, making them feel "slightly
uncomfortable."

The men confirmed after the experiment was
completed that the attractive researcher was, in
fact, attractive.


RESULTS : Unexpectedly, the men who had
received oxytocin and who were also in
monogamous relationships preferred keeping a
significantly greater distance between
themselves and the temptress researcher -- the
hormone promoted bonding with their
significant other, not the stranger.

They stayed
an average of 4 to 6 inches further back than
oxytocin-induced singletons or anyone from the
placebo group.

Finally, If you can't get your hands on
the nasal spray, there are plenty of ways in
which oxytocin release can be stimulated during
a monogamous relationship.

https://www.theatlantic.com/health/archive/2012/11/study-oxytocin-the-love-hormone-makes-men-in-relationships-want-to-stay-away-from-other-women/265314/