What is the ruling on Islamic marriage websites on the internet, and does seriousness in marriage remove the sinfulness from using them if there is sin therein?

Dealing with websites varies according to their nature and their adherence to Islamic legal guidelines. There is no harm in accessing and corresponding with serious websites that help people and whose administrators are trustworthy. As for frivolous or malicious websites, they should be avoided, and one should be wary of corresponding with them or providing them with data, especially photos, due to the possibility of them being exploited maliciously. Allah has warned us against people of desires and their wicked intentions.