Sex is beautiful is given as an excuse for sexist incidents, especially creating a sexualized environment.
The argument is that sex is a natural, enjoyable part of human interactions, and our culture unnecessarily makes sex something shameful and banishes it to certain regulated spheres. In a more free society sex would be openly discussed and celebrated, and using sexual images and metaphors in non-sexual contexts is just an example of this.
Geek feminists do not in fact argue that sex is intrinsically shameful or that our culture is sexually healthy. However, many feminists argue that active, enthusiastic consent should be the requirement for sexual interactions between two people, not that every interaction should be sexualized. See sexualized environment for some specific reasons that the use of sexual material in non-sexual contexts is criticised.
Examples
Some examples of the "sex is beautiful" excuse being used include: