Might be smell. Subconsciously he can tell by smell or something, cat (SFP), MIL(NSFP), SIL(NSFP), FIL(DNSFP), vs. fiancé (SFP)(SFP: safe for petting; D: definitely)
or perhaps even more simply:
Human smell? If yes move on, if no safe to pet
Fiance smell? If yes safe to (heavy) pet, if no, not...