Cumming inside of you

What does it feel like when your SO cums inside of you? I just got Nexplanon on May 27th and this Friday is when I was told it's safe to not use a condom. If he can come out, my fiancée has my permission to finally cum inside of me, so I'm wondering what it feels like. He said he's not going to give me a warning and his first instinct will be to pull out, so I have to keep my legs around him or be on top.