If you are a man: by wearing sandals. Of any type, that exposes your bare feet.
When you are in Spain, just notice: all Spanish men wear proper shoes, whatever type these can be, on every occasion, beaches exception. Shoes are for Spaniards a demonstration, a message to the environment confirming their masculinity, or, rather, confirming their not being a queer. By wearing sandals you send a signal that is understood only in one way – you are “el coño”.
This is a common wisdom in Spain, and it is like if they were obsessed with this matter. When they notice your presence, they eyes glide over you down to your feet to find out who you are, and they are not shy to let you know what is their attitude to you wearing sandals.
Why do I write about it? Because I recently been in Spain on a bike tripe. Spring weather, nice and warm, nearly summer. And in a climate like this I usually wear suitable cloths, and my footwear are sandals (sandals are something we in Europe, north of Mediterranean, like to wear when the weather permits it) which are also a part of my bike outfit. Not an issue in Spanish beach resorts and other touristy places where they are accustomed to habits of foreign visitors, but in provinces such abnormality doesn't escape without noticing. Sometimes comments, sometimes shouting but usually a demonstrative attention pointed against you. And if you happened engage yourself in a conversation, expect a subtle try to get prove of your sexual status by receiving a question “do you have any children?”
I am not a queer. Do you think I should wear proper shoes next time I am in Spain? :-)