If you read the conversations before that choice, Mary does mention it's just sex and nothing emotional. She suggests Charlotte explore more and she shouldn't sacrifice her sex life like that. And that, losing her husband shouldn't throw her life without fun or adventure.At first I assumed that you wanted Charlotte to become exclusive to MC instead or will Charlotte can be someone else if he choose the first option View attachment 3342679
In Charlotte's case I think it is possible to show her story that happened outside of the MC's point of view when She and Mary are telling a story with each other.
Charlotte is looking for someone who is kind, loving and caring, more of a husband. Fred pitching the idea of 'helping' her is just the sex and nothing more.