Need advice

Me and my partner have been together for 7 years. he recently told me he wants to explore abit and have some fun, he said he wants to do it with me, he wants to act out his fantasy me, him and another girl i said in the past i wouldnt mind the idea but i always kept it as just a fantasy, i never thought he would want to act it out he also said he wants to do more than that with me and explore, but im not sure as i feel its because he isnt happy with just me. i feel really lost on what to do we have recently just had our first child and i told im not sure i could do it where as he said he is definitly sure. he said he told me because he doesnt want to hide anything from me but i just feel so lost on what to do.

Has anyone else ever been in this situation that could give some advice?