Unfaithful

[Rihanna]

Not faithful; not observant of promises, vows,
allegiance, or duty; violating trust or confidence; treacherous;
perfidious; as, an unfaithful subject; an unfaithful agent or servant.

[Donald Trump]

1. Not true to ones sexual partner.
2. One of Rihannas song.

[Little_Girl]

Betraying your partner, disrespecting their trust. Being disloyal and not being honest. Cheating.

[Magaret]

When a he or she is [obviously] [sleeping around] behind their [significant] others back.