Efficient or not, depends on whom you ask.

Of course no one really knows when sex was invented but chances are it was a very, very long time ago. Possibly even before, but certainly at least very shortly after multicellular organisms evolved.

One possible scenario, proposed by Richard Dawkins, was like this.

Every organism has a special group of cells whose purpose is reproduction. These can seed a new organism on their own or need to combine with a partner. There is nothing to say what the partner must look like. In the depths of time, the partners were probably identical. Even now we have certain fungi that reproduce by combining any two spores, there are no "male" and "female" spores.

Now, due to natural fluctuations, it may just happen that some of these cells are teensy-weensy larger than others. That has an advantage of bestowing the potential embryo with more nutrients, thus giving it a bit of advantage at the start of life. But a bigger size has a disadvantage too, in reduced mobility.

But what happens if some of the partner cells happen to be teensy-weensy smaller than others? They cannot give the embryo much nutrition but can be more mobile and more easily find the oversized partners.

So we end up with two opposing strategies: "both partners about the same" and "one big and one small but mobile". Over eons of evolution, the two strategies would diversify and the "big and small" would go to extremes, with the size difference in the order if 1000:1 or even more. And thus we arrive at what we today call an "egg" and a "sperm". We may of course have organisms that produce both cells, but it is more efficient to diversify that too, with some members of the species producing only eggs and others only sperm.

The cost of producing an egg is high, which means the egg is precious and the organisms producing them must treat them as such. The cost of producing a sperm is tiny, so the organisms producing them can afford to make them in copious quantities and waste them. The rest, including the anatomical and behavioral differences between "males" and "females" follows naturally.

If this is indeed how sex evolved, then it is very likely that a similar sequence of events unrolled elsewhere in the Universe. The aliens would indeed look very different from us, but the very basic law of natural selection would be the same everywhere and thus it is very likely that they would also come in two sexes.