Someone cheated

It's cheating if your partner doesn't know about it.

It's an agreement if your partner does.

Adventurous is when you pick someone up together. 

Kinky is when everyone else is invited, too.

You're vanilla when just the usual will do. 

You're GGG--good, giving, and game--when anything (within reason) keeps it new.

Date night keeps you close. Dating secondaries keeps you on your toes.

If monogamous: love is static and finite. 

If poly: love is a fluid, renewable resource. 

If "monogam-ish", take your pick:  liquid, solid, or gas. 

Water to quench, clouds to clear, ice to preserve.