Pascal I have made the same observation and I was stunned to see how the LXX consistently translates דדיך to "breasts" rather than "love". However, the LXX in my opinion goes too far, and I cant help but think that the MT's version is superior than the LXX (in this particular instance). So while I agree that the term alone is ambiguous as @ba has pointed out, I think from the context it is pretty clear that it is to be translated as "love" and not "breasts". And I give my reasons below:
In 1:2 it is clearly the woman speaking (about her lover), so the LXX is forced to break up the verse in the middle - She: Let him kiss me with the kisses of his mouth. He: For thy breasts are better than wine. This makes the verse awkward and barely readable. Whereas according to the MT everything runs smoothly and the woman is just saying that his love is better than wine.
I think 7:12 makes no sense according to the LXX but reads smoothly according to the MT. This is the translation of the LXX:
Let us go early into the vineyards; let us see if the vine has
flowered, the blossoms have appeared, if the pomegranates have
blossomed; there will I give thee my breasts.
Breasts is not something you can give to your lover (If it means to let her lover kiss her breasts, it would still not be expressed in such a manner), whereas "love", like a kiss, is something you can give. I think this is compelling evidence that the LXX's choice of translation is inferior to the MT.
Furthermore, in the bible playing with an unmarried woman's breasts and nipples is considered a promiscuous act, the act of a whore (see Ezekiel 23:3). As the song is supposed to portray the perfect bride and groom and the paradigm of a loving relationship, it is hard to accept the LXX reading which suggests rather a licentious relationship.
The only place that the LXX readable is plausible is 4:10
How delightful is your love/are your breats, my sister, my bride!
How much more pleasing is your love/are your breasts than wine, and the fragrance of your perfume
more than any spice!
This is the only time that the LXX and MT versions are equally sustainable. So while the LXX reading is possible is some places, I think that it went too far by sticking to one reading and not acknowledging that there are other variant readings.