Is it possible to catch leprosy through occasional or sporadic sexual contacts with someone who doesn't have any sign of illness?

Very unlikely: Leprosy is difficult to transmit. It is typically spread from regular contact and most authorities feel that it spreads from respiratory droplets. It is more contagious for those people who are immunosuppressed.

Not really: it is usually transmitted human to human through respiratory droplets and about 95% of people have a natural immunity.