The claim

“Christians should not attend a same-sex wedding if they believe the marriage is sinful”

This is a normative religious/ethical opinion rather than a verifiable factual claim, so it cannot be conclusively fact-checked as true or false.

The statement uses prescriptive language ("should not") tied to a conditional belief ("if they believe the marriage is sinful"), which makes it primarily an argument about religious conscience and moral conduct. Different Christian denominations and theologians disagree on whether attending a same-sex wedding constitutes endorsement, is permissible as an act of love/support, or is inappropriate participation in what they consider sin. Because it depends on interpretation of scripture, doctrine, and conscience rather than measurable empirical facts, there is no single objective standard to verify it. A more checkable version would specify a particular church’s official teaching (e.g., Catholic, Southern Baptist, Anglican Communion), which can then be sourced and verified.