You've got it twisted a bit. She's married to someone else(in servant-land, apparently), and he's hitting her with the ntr chad moves himself. Also, if you've been keeping with the series, he's been summoning them just to film pornos of them, not the first time he's brought in some male talent.
Boudica's husband died 2000 years ago, but her servant form is still loyal to him. They used a similar concept with Archer of Inferno. Didn't stop Dorachefu from NTRing her in a recent dojin.