Any of the other girls getting pregnant?
The "(You feel like something is preventing (???) from getting pregnant!)" message happens because of a multitude of reasons.
* She doesn't love you enough (has to be at love 4)
* She's already pregnant (wait a week)
* She has a kid in the daycare (can't have more than one)
* You have not been to the nexus (tutorial girl hasn't told you the rules)
And I guess it wasn't obvious enough, the cowgirl position is a joke option.