Think carefully before you pay anything towards the agency's advertising. Ask the agency to tell you its scale of fees before you commit yourself. You might be asked for a small deposit if you take keys to look at a property. This is refunded when you return the keys. Always ask for a receipt when handing over money.
Some agencies charge a separate fee for drawing up a contract.
By law everyone should be treated equally regardless of their race or sex. Accommodation agencies and landlords are no exceptions to this rule. If you think you have been refused accommodation due to discrimination you should report it to either the Commission for Racial Equality or the Equal Opportunities Commission who will look into your complaint. Shared houses of single sex are common. In most circumstances, it is not illegal for an agency to make or continue this sort of arrangement.