"I've waited for this moment my whole life," Csincsak told PEOPLE on Friday before his bachelor party at nightclub Vanity, which followed the rehearsal dinner at Rumor.
Among the guests were Bachelor alums Vienna Girardi and married pair Jason and Molly Mesnick. Lueders originally competed alongside Molly Malaney on Jason's season, while Jason and Jesse vied for DeAnna Pappas on the Bachelorette's fourth season.
It was a fitting swap as Csincsak and Leuders attended Jason and Molly's wedding in February.
"I'm really happy for the two of them," says Mesnick. "Jesse and I have known each other for a couple of years now. From day one we said we'd be at each other's weddings and that's what we’re doing. He was at ours and we're at his."
Lueders, 28, is four months pregnant with the couple's first child as well, a boy they have named Noah. Csincsak, 28, proposed on Aug. 6.
"I tell her every day, she's the best thing that's ever happened to me," says Csincsak. "We just have fun together. We are just enjoying our time together and it doesn't matter what we do."