A Catholic couple can choose to include a first kiss in a Catholic wedding ceremony. Its omission at the weddings you attended was likely either the choice of the couples or done upon the advice of the presider. The kiss is not part of the nuptial liturgy but a social custom that the Church permits. One of the advantages of omitting the kiss is that it discourages the congregation from the unfortunate modern trend of applauding in church, which is what many people now think they should do at the first kiss.
On the other hand, some couples choose to include a first kiss because it is a social expectation that their families and guests are expecting. Including the kiss also serves as a symbol of Church teaching that a marriage ordinarily must be both ratified (through vows) and consummated (through marital relations) in order to be valid.
