Usually because of something in your domain's history, or because the software blocks adult content by design. Common causes:
- Phishing reports filed against your domain in the past
- Your URL appearing on a page where malicious code was detected
- Parental control software blocking sexual content
- Your ISP blocking sexual content
- Your domain having shared a server, and an IP address, with bad actors
There's no fast fix. You'll need to contact the company behind the warning and appeal it. We don't produce or distribute malicious software in any of our products, so nothing in your white-label itself will be triggering this.
Worth doing before you buy: check any domain against malicious URL databases before purchasing it. A clean history saves a lot of trouble later.
References: https://www.virustotal.com/