Breast Cancer

Most heterosexual men are attracted to female breasts. Breasts have various biological and psychological functions:
First of all, breasts naturally serve as a milk source for raising babies.
Furthermore, a man likes a woman who can produce enough milk to raise the children he has. So he is attracted to mature breasts. (Way back in human evolution, this was important.)

Some men also unconsciously sense that a woman with large breasts has more female hormones in her body, which cause various effects on those women, such as:
These hormones (estrogens) give women a greater need to find a potential reproductive partner <sex drive>.
Estrogens are involved in the libido (sex drive) in women.
Estrogen is responsible for the development and regulation of the female reproductive system and secondary sexual characteristics.

For example, estrogen is responsible for the development of female secondary sexual characteristics during puberty, including breast development, widening of the hips, and female fat distribution.
Estrogen, in combination with growth hormone (GH), mediates breast development during puberty and breast maturation during pregnancy in preparation for breastfeeding.

High blood levels of estrogen cause typical female characteristics to develop in young women, such as large breasts.
Large female breasts therefore have a superstimulus effect on men:
Men can see these female features from a distance in a group of people and focus their attention on a woman with prominent feminine curves.
This part of the mating ritual between man and woman has existed for tens of thousands of years with regard to visual attraction.
The man who meets such a woman with large breasts becomes attached, marries, and has children with her.
This increases the chance that their daughters will also develop large breasts.

Biological Runaway Process

This can give rise to an evolutionarily runaway growth process, as biologists call it. This means: a positive cybernetic feedback loop that increasingly reinforces the growth process.
For the last tens of thousands of years, women with large breasts have been attracting men and reproducing rapidly to give birth to daughters with—perhaps even—larger breasts, which attract even more men. Or perhaps these young ladies are more likely to attract men and marry young to have children. That is perhaps how it might have been in the past.

The abundance of female hormones in these women with round breasts likely increased steadily, after many generations (... grandmother → mother → daughter → granddaughter ...). In female offspring a risky blood level could be reached this way.
Perhaps too much estrogen causes the mammary glands to expand so rapidly that they can cause breast-cancer (instigated by a runaway process of cell replication). Scientists have found that estrogens play a role in hormone-sensitive breast cancers.

Thus, men's lust for very large breasts (superstimulus) can indirectly cause cancer in the daughters of every new generation, or later in future generations. When such an evolutionary trend is present in a particular lineage, it is necessary to seek a balance between lustful love and physical health.