In Sonnet 142 (which is commonly linked with Sonnet 141), the speaker addresses what critics have called the "Dark Lady," a woman the speaker loves; he challenges her to return his love, and supposes she is having other affairs. 

The speaker loves the Lady; this is his "sin." The Lady's (thy) virtue is hate. She hates the speaker's sin (which is loving her) because her sin is also having other, presumably extramarital, affairs. Comparing his sin and hers, the speaker determines that his is not worth reprimanding ("reproving"). Even if it was worth condemning, the Lady is as guilty, if not more so, and therefore, she is in no position to judge. Her lips have "profaned their scarlet ornaments" and the scarlet refers to the red worn by cardinals; her lips have profaned their moral appearance and she has "sealed" superficial love as often as the speaker has. 

He loves the Lady as she loves her other lovers. As she looks to seduce them, the speaker looks to seduce her; this is the gist of lines 9-10. He then asks her to pity him because she looks for pity herself. The speaker indicates that his Lady, in sleeping with other men, is secretly craving pity. If she secretly craves pity then she should return the favor and pity him. Otherwise, she should deservedly be denied such pity: 

If thou dost seek to have what thou dost hide,
By self example mayst thou be denied! 

Here, to take pity on someone means precisely that, but it is combined with a sense of taking pity to the extent of showing affection and possibly reciprocating love or sex. Essentially, the speaker says that if the Lady is going to share her love with others, why not share it (and/or pity) with himself.