St Thomas' argument is very dependent upon the definition of "the honesty of marriage" which wasn't mentioned. Either way, the Church's teaching is clear from C.C., so I don't have to worry about it.
As I was always taught, "just because something can be done, doesn't mean it should be." This concerns imperfections, not sins. Geremia, if you want to argue that there is a greater 'perfection' in acting a certain way, in the situation of pregnancy, that is fine, but you can't ADD sin to a situation already decided upon by the Church. That's scrupulous.