Libido is not related to viagra -- only performance. It does work on women, too -- increasing blood flow in significant places. Even in men, it doesn't increase sex drive -- just the ability to do one specific thing. It doesn't create interest in doing that thing.
Libido on the other hand is about all kinds of things, from feelings of affections (or alienation), comraderie (or resentment), to energy (or exhaustion). People who are tired often don't know, or if they did know they don't remember, that if they have sex even when they are tired, they're now still tired, but also satisfied and happy...
Dr. Ruth used to suggest that as long as you are not repulsed by the idea, loving partners do loving things for each other, without everyone needing to be 'in the mood' every time. If he wants to have a go, unless you have a VERY good reason not to, you may find that after the first 5 minutes or so, you ARE in the mood, even though you weren't initially.
Kind of like not really minding if your husband does the dishes to help out, even if he's not really in the mood to...
The interesting thing about sex is that the people who have the most want the most and end up having a much higher libido than people who wait for their libido to get higher by not having any at all... The way to increase a sex drive to do sexy and sexual things, and wait for the feelings to follow.