As far as Sonnet 116 is concerned, loyalty plays a key role in true love – actually, the only significant role. The poem asserts that the true marker of love is its persistence; without constant devotion, "love is not love." A lot of difficulties can arise when two people who love each other, but if their feelings are real, none of these things should matter. In the ideal world of the poem, true lovers always forgive each other and stay together, regardless of the circumstances.
Sonnet 116 suggests that true lovers remain loyal, despite any "tempests" that may strike their relationship. This includes even, paradoxically, infidelity.