Why is birth control aimed towards women and male enhancement aimed towards men?

I tried to word this as best as I could. Why are women expected to take birth control to keep from getting pregnant and "trapping" the man? Why are men encouraged to sleep around with a big slong, thanks to Viagra and that stupid commercial with the smiling guy?

Update: Why isn't the opposite? Why don't men take a daily pill to help prevent pregnancy and why don't women take any sexual enhancement drugs?
Update 2: Read the entire question.
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