Historical Romance - MMC Teaches FMC Pleasure

I can't remember the name of this book I really liked, and it's been bothering me for a year now. I checked out the other posts related to this trope, but I haven't found anything that's quite right yet.

In the book, the virgin FMC asks the MMC for lessons. MMC is staying with her family when the lessons occur (I think he knows FMC's brother/family). He agrees, but only because he doesn't want her to find someone else to teach her. He's very hesitant/cautious about the lessons and goes very slowly, because he doesn't want to ruin her. One of their earlier lessons occurs outdoors (they don't go all the way in this lesson). There is a third act separation, because the MMC is forced to leave and address an emergency (possibly related to his family). The FMC might find out she's pregnant after he leaves? MMC eventually returns and grovels a bit, and they marry.

I remember the MMC is a duke or marquess or something like that, so the book is probably regency, but I can't be certain.