casual sex is a weird idea to socialists, because it's almost impossible to understand it happening in the absence of coercion. marx famously argued that all sex under capitalism is prostitution. a more contemporary analysis may suggest that the ubiquity and normality of casual sex is just a reflection of the impulsiveness of capitalist society, a way to fill a void left by the emptiness of consumerism, and would largely whither away in the absence of the capitalist relation.
i wouldn't have articulated myself like this at the time, but i don't think my feelings abut the topic have changed much.
so, we're not going to actively prohibit it, the way conservatives will. you can have your meaningless fucks if you really want to. but, we're not going to promote it or live it the way liberals do, either. what liberals call "sexual freedom" is not some kind of emancipation or something, it's just kind of matter-of-fact.
socialists will mostly largely argue for a society where sex is kind of an irrelevance - a necessity to carry the next generation forwards, and a valid expression of emotion when appropriate, but not this pre-occupation that exists as a distraction from more interesting things, the way it is in contemporary capitalism. we'll deconstruct the status quo as more or less a control strategy, and even break apart a lot of the existing narrative as divide and conquer. and, we'll argue that once you're actually free, once you can actually pursue things that are meaningful and substantive, that you'll get bored with casual sex pretty quickly - that you'd rather find somebody to love.