Makoto is in a pitiful situation. Her husband (a non-entity in this story) was not married for love but for "why not". Her first sex was with her husband and she found that she loves it. Now she's not getting any. There was a hint that he could be cheating on her (Chapter 70, Page 20 "He's been doing this a lot lately...") even though that really doesn't make sense since it appears that he doesn't even WANT to have sex (unless he finds her repulsive because she's too loud during sex?).
Makoto's husband doesn't fulfill her needs either emotionally or sexually. He also doesn't treat her very well (example Chapter 54, Pages 2 and 3). Aki really does make her feel loved. This is clear in several cases where she's feeling depressingly alone and then he's there (such as the heart-crushing "vacation" and his appearance in Chapter 39, Pages 16-20; Chapter 41, Pages 1 and 2), but can really be seen when he's taking care of her when she's sick as nobody else has ever done (Chapter 54, Pages 14 and 15).
Aki and Makoto have already cheated in just about every way short of oral sex, genital-to-genital touching, and intercourse. Makoto wishes she had done more (Chapter 46, Pages 16-19) while Masami's attempting to NTR her.
The "happiest" situation that could happen for these two to finally become a legit couple would be for her husband to divorce her for his own reasons unrelated to Aki.