Throughout this sonnet, there are a few words with multiple meanings:
Lie(s) – Line 2 says, “lie(s)”, which can either mean not telling the truth or during that time, sleeping with another lover. On the second to last line, Shakespeare writes, “Therefore I lie with her and she with me,” which in this case can be about them lying to each other, or how they remain a loving couple. On the last line, “And in our faults by lies we flatter’d be,” I feel like “lies” means to not tell the truth, because of the word, “faults” before it. The two use their lies to balance out their relationship. I just want to point out that on the first line, he calls his lover “my love” showing they stay together, despite the lying.
Vainly – The next word with two meanings is, “vainly”, which is on line 5. In one case, it means that she just thinks he is young and inexperienced, or it could mean that she does not want him to learn about her infidelity.
Simply – The word “simply” is located on line 7, and the two meanings I pulled out of it are something made solely up of one thing, or something easily done. In this context, I think the meaning would be the latter.
Habit – The last double meaning word is “habit”, on line 11, I found very difficult to grasp. The only meaning I found other than something random (a loose garment) and a regular practice, which seems to make more sense. The line would explain how love is best seen is though trust.