Whenever I think about writing a thesis statement for a paper I look at the literary work and list the things that I know for sure that I understand (because that is what I will cite as evidence later on, too). First of all, look at your assignment and see if there are any specifics that the instructor is looking for you to understand. For example, are you supposed to write about the figures of speech used in the poem or argue a point based on Shakespeare's thoughts or philosophy that comes from the poem? Then, remember that strong theses have strong words like "should" or "must" or "is". The following are some thesis statements that I thought of that might help you:
1. According to Shakespeare's Sonnet 116, Love is "an ever-fixed mark" that should never be withdrawn.
2. Shakespeare believes that one should never give up on love because it is stronger than Time or anything else that might get thrown at it.
3. Time, faults and life's trials cannot destroy true Love.
Focus on the language that you do understand and back your thesis up with that evidence from the poem.
I described love's poetry and the strength and sanctity of marriage, as I too am married. However, my thesis wasnt too clear