I really think that Zoro banter with him subtly taught him the security that love can bring... so how could they not love each other
They even did not need promises, just maintaining their playful and engaging daily routine was the best promise they could make
agree with a netizen who say that neither combat power nor authority can replace emotional security
External forces can be strengthened through cooperation and competition, emotional security can only come from love
Sanji loving everyone of the same gender won't bring a sense of security, Zoro often brings security cuz this guy takes promises most seriously
Sanji has suffered a lot of trauma since childhood and his love is actually a form of self-protection