For years, Julie Brinton’s days have been three young children through dinner time, bath time, and bedtime. What do you think she wants by the time she crawls into bed each night?  She has one thing on her mind only: TV.

But some nights, her husband, Rob, reaches over to rub her shoulders and offer her a back rub

And then Mrs. Brinton thinks: “Has it really been three weeks? I guess we should probably have sex.”

“I will do it for him,” says Ms. Brinton, 34, who lives in Mesa, Ariz.

Mr. Brinton, also 34, appreciates his wife’s gesture. “But afterward,” he says, “I always feel guilty, that I’ve been selfish.”

Research from the University of Toronto shows that the reasons why partners have sex in the first place also significantly affects marital satisfaction.

For many years, scientists believed that humans had sex for a few simple reasons: to reproduce, experience physical pleasure or relieve sexual tension. Then a 2007 study from the University of Texas identified 237 expressed motives for sex. The reasons ranged from the mundane (stress reduction) to the spiritual (to get closer to God) and from the altruistic (to make the other person feel good) to the spiteful (to retaliate against a partner who cheated by cheating).

Now, two studies by University of Toronto researchers published this month in the journal Personality and Social Psychology Bulletin have divided the most common reasons why people have sex – and the ones most relevant to long-term relationships -into two broad categories of motivation: approach and avoidance. Approach motives pursue a positive outcome (“I want to increase intimacy with my spouse” or “I want to feel closer to my partner”). Avoidance motives aim to evade a negative outcome (“I want to avoid conflict” or “I don’t want to feel guilty”).

Mrs. Brinton thought, “I want to enjoy sex. I want to feel connected to my husband. I want to reclaim my sexuality.” She bought new lingerie and started reading erotic romance novels. Mrs. Brinton also asked her husband to go to a sex therapist with her.

Her husband says he was thrilled. He figured there would be a lot of sex as homework. But, at least initially, their homework was to focus on real communication – not just small talk – about issues unrelated to sex. “I came to realize that you can’t have a great, intimate sex life until you have learned to connect outside of the bedroom,” says Mr. Brinton, who owns a custom-framing business.

Once “we knew how to talk about other things, we felt comfortable with the difficult questions about what the other person likes in bed,” says Mr. Brinton.

They say they are both careful to focus on feeling good. “Every reason we have sex now is a positive for me,” says Mrs. Brinton.