"But is Tomas really happy though?"
Hmm. I have to fill in the blanks to a certain extent in order to interpret the question so that I can answer the question. That obviously carries the risk of answering a misinterpreted/unsolicited question, but I'm doing the best that I can.
What I'm hearing now from this question in hindsight is the question of whether or not Tomas is satisfied with a life of relative celibacy with Tereza in the countryside after spending so much of his life carrying on as a highly sexually active polyamorist (or womanizer or slut or however you prefer to categorize his lifestyle). It's the question of whether or not he can radically change his sexually gratuitous habits and mindset and still be content.
So, I think there are some preconceived notions that we need to break down here. Even if these preconceived notions don't exist, we still need to chop them down preemptively. The first is that Tomas' sexuality is intrinsically tied to his identity, or personality, or purpose. Kundera makes a point of illustrating Tomas' solid and unambiguous distinction between love and sex. Tomas' philosophy within his sexual escapades is that he can easily uninvest, can easily attach and detatch himself unemotionally from his other sexual partners. This lends itself to the wider example of being able to detatch himself from all sexual infidelity entirely, which is what he does in theory on a microcosmic level every time he returns to Tereza.
The second notion is that his sexuality and libido remain at a constant. I can see where it would be difficult for a 20-something-year-old to even conceive of a life where sexuality just wasn't that important, but as Tomas enters the latter stage of his life his perspective, priorities, and desires naturally change. Tomas goes through a lot in his middle-age years, dealing with being politically pigeonholed and ostracized by communist Czechs, dealing with the awkward relationship with his estranged son who has become politically active himself, dealing with Tereza's unstable psyche, and just dealing with aging in general, and the ways in which his biology and biochemistry naturally change. It's reasonable to conclude that Tereza's difficulty in coping with Tomas' infidelity had a direct effect on Tomas' change of nature. But I believe that the greater reason for Tomas settling down has to do with the fact that it was just a natural consequence of the stage of life that Tomas found himself in. By the time one reaches their 50s and 60s it's common to want to move out to the country, to a quieter simpler life, to want to ease into a monogomous lifestyle filled with routine and stability, to want to "settle down" as it were.
This ties in closely to the third notion, which is that happiness is an attainable state in the first place. Life wears you down. The world wears you down. By the time Tomas and Tereza moved to the country, I don't even think they were seeking happiness anymore. I think they just wanted to live. To live with their own intrapersonal and interpersonal flaws and conflicts, to live with their accomplishments and disappointments, and to support one another. To live is not always goal-oriented, and certainly not oriented upon the nebulous, ephemeral goal of "happiness." Survival itself is the goal. Partnership is the goal. To be, or not to be.
Tomas wasn't sacrificing any part of his lifestyle or of himself to grow old with Tereza. I feel that at all times in the story Tomas was constantly looking out for his own best interests, which includes his love of Tereza, and which also includes his unwillingness to compromise in his sexual adventurousness at the time in his life when it meant something to him. But things change. Listen--that's what's there. That's the most important thing you can learn from the air, from the wind, from lightness: things change.