The husband-and-wife scenes are made less exciting due to being briefer and not including animations. The only time that sex is animated on any route is when Martin is having sex with Emma, and even in the "threesome" scenes, the only part that's animated is when it's Emma and Martin; when it's Patrick and Emma, there is no animated component, and as you noted, Martin gets about 80% of the action (including uninvited anal). Emma and Martin are even animated while exercising together! Also, if one sticks to the "vanilla" path, there are altogether less sex scenes. Thus, the "vanilla" route is placed in a position of being less interesting due to the sex scenes being briefer, less frequent, and less animated.
The moving-in threesome creates an especially stark contrast: By the time her husband walks upstairs to find them on moving day, Emma has already given Martin a vigorous blowjob downstairs and then is in the middle of receiving vertical cunnilingus from Martin in their upstairs bedroom. So despite never having sexually interacted with another man outside of her husband's presence, she can't wait until he gets back from returning a rental car to start a sexual encounter with Martin.
And yes, even on the monogamous path, Emma is walking in her underwear in the hallway (and falls in front of Martin's dick), peeking into Martin's bedroom while he masturbates, wearing a skimpy bikini for a photoshoot, allowing Martin to place his dick between her ass cheeks without saying anything, and getting into a hot tub with two men naked, none of which she shares with her husband. Then after all that, Emma again leaves their bedroom at Martin's home only in her underwear and stops to converse with him in the hallway as he's obviously gawking at her and comments about how lucky he is to have caught her dressed like that (which who knows how long would have continued if not interrupted by Patrick).