I'm not sure if there's been a Papal decree praising it as moral, but it is certainly in line with the Catholic teaching on sex. As others have said, it assists sex in its natural form.
Sex must take place within marriage, and the formular of marriage is procreative and unitivite. Viagara assists both procreative and unitive. It doesn't change the sex act, it doesn't remove it from the confines of marriage and into a lab, it doesn't change the body to frustrate the procretive and by assisting the man it helps the unititive.
Viagara, if anything, is morally neutral; inof itself, it is not inherently evil nor inherently good. It can be used for evil, sure, 50 year old wanting to get it up so he can do his 28 year old secretary - evil. 50 year old man needing the assistance for celebrating his 25th wedding anniversary with his wife - good.
Seeing as the only impediment to Catholic Marriage is impotence, this can help men who otherwise would find themselves facing annullment proceedings.
I understand what your saying but a part of me is still unsure. I think I will read over the Church's teaching on sex. Also, all "good things" do come to end.