In the past and even today one can see marriages being done either for political or economical purposes without the “full” consent of the couple. Were and are these marriages valid to God? It is no wonder why adultery creeps in because the spouses do not really love each other, so they must have a private lover to share their lives.
Marriages are presumed valid unless proven otherwise by legitimate Church authority. In modern times, ordinarily that is a diocesan marriage tribunal. It is true that unwillingness to marry can invalidate marital consent, but such a case must still be reviewed by a marriage tribunal before invalidity can be declared.
In any event, this is of secondary importance to these more serious assertions: That being forced into marriage allegedly legitimates adultery and that unwilling spouses must find a lover with whom they may share their lives. Both assertions are incorrect.
Even in the past, most people were not forced into marriage, if by that you mean that they were prodded to the altar at swordpoint. More usually, families would press their children to marry well and would usually have prospective spouses in mind for them. Sometimes families, particularly noble families, would also promise their children into marriage to establish alliances. But if a child was truly unwilling to marry (either altogether or one particular person) that child ordinarily had recourse to the Church to either protect him from marriage or to dissolve an invalid marriage. In no case did a marriage arranged for temporal advancement or a forced marriage legitimate adultery.
It is also false to assert that adulterers are “sharing their lives” with lovers. In fact they are using other people to satisfy lust while denying them the true “sharing of lives” that comes with marriage. What they should in fact be doing is to cultivate their relationship with the spouse that they have and will to love that spouse, despite any current lack of romantic feeling.