I agree to a certain point. Rika's goal is true/ultimate love, finding her soulmate. It's hard to guess what the author's belief on love is. It's debatable. Love views vary and in today's world, it can boil down to the feelings you have for a person.I'm not sure if the story supports it or not, but I wouldn't be surprised if Rika never had sex with anyone else since marrying Sousuke, and that everything was part of her plan. Maybe Shingo was drugged himself and he only thought he was having sex with Rika. Because after all the twists and turns turns the mangaka might thing that the best plot twist would be Rika having been "faithful" to Sousuke all along...
And, as far as I can remember, Rika was never shown explicitly fucking someone else...
Those butterfly's in your stomache, always thinking about that person. And in some cases those feelings are mutual and so people get married because of those feelings. But those feelings of "love", honeymoon phase, and infatuation don't last forever.
-Even in a marriage you can experience the same feelings of "love" for a person outside your marriage/spouse. The most dangerous emotion that leads to most damaging affairs is called Limerance. You put yourself into a strong delusion that prevents a person from thinking logically anymore. Nothing matters anymore, except your fantasy (unrealistic) life with your affair partner. Most romance stories that glorify/justify affairs are always fantasy/delusions people come up with and 95% of the time, never truly lead to a happy ending in real life. Most people, after commiting the deed, get shocked out of limerance (grass wasn't greener on the other side) and immediately regret it.
Now I'm going to explain what love is according to the examples we see from Jesus's Christ's Teaching/example of love from the bible [Note: I'm not here to debate if God is real or not here. This is just for reference from a religious point of view of what love is].
-Love your neighbor as you love yourself. Treat your wife, children, family, friends, and strangers as you would want other people to treat you (with love). Helping one another. As much as you would hate someone to lie, curse, and betray you....you would do your best to not do that to others. That is loving people.
If you catch feelings for someone (Co-worker, family in-laws, your neighbor ect), because of your love for your spouse/girlfriend/boyfriend/friends, you would cut/minimize contact with that individual because you are not willing to hurt your spouse or destroy your family because of your selfish desires. Just as you would want your partner remain faithful to you.
Love in Marriage is commitment to stay with each other during your best times and your absolute worst times (If it's physical/sexual abuse get out!). Until Death does you apart. It's a promise/vow you make to each other on your wedding day.
While it would be a huge plot twist (I also thought of this) that Rika never had sex outside of her marriage (as of this chapter even while engaged) with Sousuke. This would turn Rika into a Yandere character who's sole devotion and obsession was to guarantee that Sousuke would never cheat on her, even to the point if he was ever drunk/drugged. Because Sousuke would only sexually desire Rika but that would also mean Rika (Yandere) is only sexually attracted to Sousuke and cannot have sex with other people neither. Or her tendencies make her capable to have sex with others if it helps her goals to ED sousuke from other women.
Narcissists have a distorted/selfish version of attachment rather than empathetic love. Which is why I think Sousuke's story might be how to escape a toxic marriage that involves children. And the mental repercussions he will have once leaving it.