You are of course correct, but you must remember that these theories are not looking at whether these effects are as potent in the present day as they were in the past when we first evolved to do them, they're looking at why we originally evolved to kiss and how that translates to the instinctual driving forces of why we kiss now. As nobody knows when we even started kissing, obviously it's difficult to do this.

But the theory is based on the fact that kissing seems to be a way of sex selection in women based upon immune systems, as men don't experience the same process, and appear to more treat kissing as a means to sex, then it is likely that it was the male desire to induce sex that first brought kissing about. So the theory is that men began kissing women because they realised it could induce arousal, and the belief is that this is likely linked to testosterone.

In terms of why this theory is so widely accepted, there are a lot of factors that when combined do make this theory make a lot of sense. For example, it is often forgotten that women technically have a period where they go into heat, women's testosterone levels are higher in September through to December, and at the lowest during the summer months. Inevitably, we know that women are not bound by this anymore and have not been for a long time. But it still exists. This implies that the female sex drive of our ancestors would have been a lot lower, and so likely far more susceptible to testosterone. As the males likely had higher levels than they do now, it is likely that through kissing they could have provided enough of a boost to arouse the females of the time.

Also, the fact that women typically avoid kissing if they decide a person is not a match i.e. the kiss does nothing for them, it does give further credence to the idea that kissing likely once had the power to induce a higher level of a desire for sex in women through testosterone sharing alone i.e. before we became an intelligent species.

Of course nobody can say for certain, and things like this will always be just theory. But it does make a lot of sense based on what we know. And if it was the case, it would likely mean that the passing on of testosterone from men even in low doses to women through kissing likely does increase the desire for sex, though only if it is with the right man — or women — because inevitably the dose on its own is no longer enough to have an effect.

So it is likely now more psychological factors which detect what is going on through the kissing, but even still it would still mean that the desire in men for kissing stems from the desire to pass on testosterone to boost the sex drive of the person they are kissing, and the desire of women to stop kissing if it is not the right person is born from a defence against this effect.

But who knows, it is certainly a fascinating theory though! Thanks again for reading :-)