Husband wants to watch his wife have sex with anot

we are Just a young shy couple curious about swinging and cuckold sex.
Him fantasises about her having sex with others and having sex with other women, she gets wet thinking/fantasising about other men fucking her whilst he is involved in some way or another.

Only at a fantasy stage at the moment.

would like advice and/or tips on how to get wife to want to seek/engage in swinging and cuckold type sex.
So far we are only roleplaying and sometimes discussing the idea/possibility, this has been going on for sometime now and husband would like it to proceed/grow into something a little bit more naughty and adventurous

Update 9/6/2015: wife has now mentioned to me she has been thinking of trying our fantasy of sharing her with another