It is not a simplistic issue of either being ambivalent or being decisive. The crucial point is about what things it is wise to be ambivalent and about what things it is wise to be decisive. For example, it has been common now for decades in the U.S. for the children of broken families and others to be highly ambivalent about marriage. Another recent media story noted that the number of unmarried young adults is at an historic high due to economic conditions, but more fundamentally due to a long-term trend favoring sexual cohabitation.
Many are highly ambivalent about marriage, regardless of economic conditions, because they are extremely cynical and pessimistic about love and commitment. Yet, when it comes to having sex, the ambivalence flies out of the window--on the issue of having sex outside of marriage, decisiveness comes through the bedroom door with both feet.
The same individual is ambivalent about some things but decisive about others. We are now and have been for years in a culture that is very ambivalent about marriage and quite decisive about having sex whenever with whoever happens to be conveniently at hand. The non-marital sexual aspect is so "decisive" in character that it is now beyond the realm of even being a conscious decision or issue--non-marital sex is simply the unconscious, unquestioned default mode like eating and breathing.
Now, is that the type of "decisiveness" that is of value? Not at this blog. Sex without permanent commitment cries out for ferocious ambivalence and hesitation. Just ask those who have endured single motherhood, abortions, and promiscuity (whether serial or simultaneous) with their profound life-long emotional, psychological, and even physical repercussions. Sex outside of marriage is a cliff at which it is better to come to a full stop and peer first over the edge with great concentration of mind.
On the other hand, marriage should be the default preference because of its obvious focus on commitment, teamwork, exclusiveness, intimacy, permanency, and security. Yet, many remain ambivalent about these goods while decisively going right off the extramarital sexual cliff. (The issue is further complicated by the fact that the regnant, decisive embrace of non-marital sex actually makes people less marriageable, which in turn further bolsters ambivalence about marriage. There is a mutual, feedback relationship between the two social realities of marriage and non-marital sex.)
Again, it is not a superficial question of being either decisive or being ambivalent. It is rather a matter of choosing when to be decisive and when to be ambivalent. But in order to be able to pick the right approach of either being ambivalent or being decisive for a particular situation, it is not unhelpful to have a moral code and a fund of practical wisdom. Without such a moral code, without such wisdom, we get irrational ambivalence and irrational decisiveness, both of which lead nowhere.