The original French text is:
Jusqu’ici chaque fois que Mersault avait lié avec une femme les premiers gestes qui engagent, conscient du malheur qui veut que l’amour et le désir s’expriment de la même façon, il songeait à la rupture avant d’avoir serré cet être dans ses bras.
My literal translation:
So far each time that Mersault had associated with a woman the first acts that commit, aware of the misfortune that love and desire express themselves in the same way, he had thought of the break-up before having clasped that being in his arms.
(Comparing this with the published English translation (by Richard Howard), I think the latter has mangled the first clause (‘lié avec’ means ‘tied/linked/associated with’ and not ‘spent time with’) and introduced the word ‘must’ into the second clause. But neither of these defects affects the answer to the question in the post.)
The contrast here is between romantic love (‘amour’) and sexual love (‘désir’), both of which can be expressed by the same ‘gestures of commitment’, for example taking someone in one’s arms. Mersault describes this as a ‘disastrous fact’ because the ambiguity leads to miscommunication: when Mersault expresses his sexual desire in this way, the woman misinterprets it as a gesture of romantic love and commitment, and this later causes her unhappiness when Mersault ends the affair.
But this is disingenuous: it is not the case that love and sex must be confused. Mersault could, if he wished, use his words to disambiguate his feelings. We deduce that he does not, because it is convenient for him to allow the woman to be deceived into believing that he is in love with her.